Prior to his departure from Soul Society, Aizen appeared as a mild-featured man with brown scholarly hair enhanced by square glasses, along with soft brown eyes. He wore the standard Shinigami uniform with a captain's haori. During his ascent to Hueco Mundo, he removed his glasses and swept his hand through his hair, revealing menacing eyes and a very different hairstyle; his hair is now swept back, with a strand hanging down his face.
In Hueco Mundo, Aizen initially wears Arrancar clothes over his shihakushō. After Orihime Inoue is taken to Las Noches, Aizen wears full clothing similar in style to the Arrancar, complete with a pink sash around the waist.
Upon his defeat by Ichigo Kurosaki, Aizen loses all of the physical changes made by his transformation and returns to the physical form he had prior to transforming.
Aizen is a polite, soft-spoken intellectual, often addressing his subordinates by their given names. He rarely shows any sign of alarm or distress, and often has entire situations planned out well in advance. Whenever confronted, Aizen is casual and uncaring, attempting to draw out a conversation and make small talk, which often infuriates those he is talking to.
Aizen initially appeared to be a very kind, well-respected captain and was looked up to by many, especially his lieutenant, Momo Hinamori. However, this was just a mask to disguise his manipulative, and very dangerous, nature. In truth, he cares very little for his underlings, and is willing to use almost everyone around him as pawns to achieve his goals. He is willing to stop at nothing to further his selfish ambitions, including manipulating or killing anyone if he deems it to be necessary. He frequently toys with his underlings for amusement.
During his time as a captain, Aizen liked tofu, but he disliked boiled eggs. In his free time, he often read, and he gave a special lecture of calligraphy at the Shin'ō Academy because he excelled in this branch of arts. This optional course was immensely popular among the Shinigami students, and many of them felt great sorrow at Aizen's sudden departure.
As the commander of his army of Arrancar, Aizen controls his minions through acquired respect or fear. Ulquiorra Cifer revealed to Orihime Inoue that they are all there to further Aizen's ambitions. Aizen keeps more rowdy individuals like Grimmjow Jaegerjaquez in line through intimidation and mind games. The Arrancar Aisslinger Wernarr claims the Arrancar follow him because he has no fear, something which beings born from fear and despair find inspiring.
Gin Ichimaru has some sense of Aizen's emotions and feelings, correctly pointing out Aizen was enjoying the invasion by Ichigo and his friends and their successes within Las Noches, which Aizen himself confirmed, and noting Aizen was surprised by the arrival of Momo during the battle of Fake Karakura Town. In contrast, Kaname Tōsen has shown no such understanding, and has fallen victim to one of Aizen's more minor mind games when Aizen subtly manipulated him into punishing Grimmjow for him while Aizen himself kept up his appearance of benevolence.
Aizen's true objective was to overthrow the king of Soul Society. As he and his fellow traitor captains were departing from the Soul Society, he told 13th Division Captain Jūshirō Ukitake "no one has stood on the top of the world", not even the gods, but he aspires to "stand on the heavens and end the unbearable vacancy on the world's throne".
According to Kisuke Urahara, since mastering the Hōgyoku, Aizen has become less cautious in battle, for Aizen would not have allowed Urahara to use level 90 Kidō before he subdued the Hōgyoku. Aizen admits his new powers allow him to let his guard down. Due to this newfound power, Aizen has developed an arrogant and overconfident attitude, often not even thinking his attacks or defense through any longer like he used to, or even bothering to find out the effects of his opponents' and his own attacks. In his newest form, he has developed a severe superiority complex, believing he is now above both Shinigami and Hollows. Because of his superior attitude and high intelligence, Aizen is easily enraged when facing something or someone which he does not understand; for example, when witnessing Ichigo Kurosaki use Saigo no Getsuga Tenshō, Aizen becomes furious because he is unable to comprehend how a Human has surpassed him.
At an unknown point in the past, Aizen became part of the 5th Division under Captain Shinji Hirako. He invested himself in the research and creation of the Hōgyoku even before Kisuke Urahara. To this end, he sacrificed hundreds of Shinigami and hundreds of Rukongai citizens who bore Shinigami talents to his own Hōgyoku, but it was not satisfied with this. At one point, a young Gin Ichimaru watched Aizen meet with three other Shinigami who were kneeling before him. One of the Shinigami handed Aizen the Hōgyoku, prompting Aizen to put the orb in a glass and hand it back to the Shinigami. Aizen and his subordinates left an unconscious Rangiku Matsumoto a short distance away. Aizen eventually caught Shinji's attention, and was promoted to a lieutenant in order for Shinji to watch over him closely, but Shinji's action only helped further the realization of his plans.
110 years ago, Aizen made his way to the 5th Division living quarters of his captain. He and Shinji discussed style and trends before they made their way to the promotion ceremony being held at the 1st Division headquarters. As they got there, he witnessed 12th Division Lieutenant Hiyori Sarugaki attack her captain, and watched as the two got into a childish argument. He followed behind while the other captains congregated and had a brief discussion on the status of the Gotei 13. Overhearing 8th Division Captain Shunsui Kyōraku mention that the previous captain of the 12th Division was promoted, Aizen interrupted and asked Shunsui what he meant by this. When Aizen initially conjectured that the promotion was to Central 46, Shunsui explained it had nothing to do with the Central 46 and revealed the previous 12th Division captain, Kirio Hikifune, had been accepted into the Royal Guard, shocking Aizen.
The following night, after Urahara's appointment as the new captain of the 12th Division, Shinji had a short talk with him about his new role as captain. While leaving, Shinji revealed Aizen, who had been concealed behind camouflage. Complimenting his captain, Aizen asked Shinji when he sensed him there. Stating he had known ever since Aizen was in his mother's womb, Shinji ordered Aizen to follow. Noting his disapproval, Aizen commented on how scary a person Shinji was, prompting Shinji to say that Aizen was the scary one.
The next night, Aizen saw Gin killing the 3rd Seat of their division. Informing Gin he was impressed by his skill, Aizen asked him how he found the fight, prompting Gin to state he found it pathetic.
9 years later, Shinji and Aizen were walking around the Seireitei when they came across Urahara, Hiyori, and 12th Division 3rd Seat Mayuri Kurotsuchi. While Shinji once again engaged in a childish fight and argument with Hiyori, Aizen asked Urahara if he had heard the news. When Urahara told Aizen he had not, Shinji told Urahara about the series of strange deaths which had taken place in Rukongai and informed him the 9th Division had been sent to handle the investigation.
With the disappearance of the 9th Division, Captain-Commander Genryūsai Shigekuni Yamamoto called an emergency meeting and assembled a team to investigate. After coming into contact with Hollowfied 9th Division members, the investigation team was dispatched by an unseen assailant. While the investigation team was still out searching, Shunsui and some other Shinigami on guard saw someone whom they believed to be Aizen.
It was later revealed 9th Division 5th Seat Kaname Tōsen was behind the betrayal of the 9th Division, having stabbed Kensei Muguruma in the back and killed Heizō Kasaki, Shinobu Eishima, and Tōdō Izaemon, who had accompanied Kensei. He subsequently subdued the team sent to locate the missing 9th Division members using his Bankai. When an injured Shinji asked Tōsen why he betrayed his captain, he was surprised to be answered by Aizen, who stated Tōsen did not betray anyone and faithfully followed his orders.
When Shinji admitted he was not as surprised as he seemed to be, Aizen asked Shinji if he suspected him and stated that he had expected this much from his captain. Explaining how he had known something was dangerous about Aizen and he could not be trusted, Shinji said he chose Aizen to be his lieutenant so he could keep an eye on him, prompting Aizen to state that he was grateful because Shinji did not suspect a thing due to his deep-seated doubts. When Shinji said he already made it known he had suspected him, Aizen reveals Shinji did not realize Aizen was not the one walking behind him for the past month. Explaining the power of his Zanpakutō, Kyōka Suigetsu, Aizen informed Shinji he would have been able to see through Aizen's deception by picking up on subtle differences between Aizen himself and Aizen's double if he was as close to Aizen as other captains were to their lieutenants, but it was his lack of trust which kept them from getting close and allowed Aizen to fool Shinji. Stating he could have always refused to be Shinji's lieutenant, Aizen revealed he decided against this action because his captain's skepticism made the position ideal to carry out his plans. When Aizen stated Shinji was to blame for his current predicament and his comrades' plight, Shinji was provoked by this, triggering a speed-up in the Hollowfication process. Thanking Shinji for being so easily provoked, Aizen noted the observed connection between Hollowfication and agitated emotions. Ordering Tōsen to cut down 12th Division Lieutenant Hiyori Sarugaki, Aizen thanked Shinji for helping him and raised his blade overhead in preparation to cut Shinji down, only to be attacked by Urahara, who cut Aizen's lieutenant badge off.
Aizen and Urahara had a brief discussion about what was transpiring, with Aizen acting innocent at first until Urahara noted the investigation team did not have simple injuries, but were in fact undergoing Hollowfication. Commending Urahara on his deduction, Aizen revealed himself to be behind the disappearances in the Rukongai. Leaving the scene of the crime with Gin and Tōsen, Aizen said that there was nothing more to do there. When Tessai fired Hadō #88. Hiryū Gekizoku Shinten Raihō at them, Aizen blocked it with Bakudō #81. Dankū, allowing them to escape.
At some point after the Hollowfication incident, Aizen became captain of the 5th Division, and eventually revealed to Gin the secret to avoiding the complete hypnosis of Kyōka Suigetsu. He later made Gin his lieutenant. Continuing to experiment on Hollows to transform them into Shinigami, Aizen created Hollows which were close to Shinigami, such as Metastacia, and Menos capable of concealing their Reiatsu. While testing out this breed of Menos, Aizen and Gin saved the then first year academy students Momo Hinamori, Izuru Kira, and Renji Abarai from them. With his interest piqued by them, Aizen realized their potential and took them all into his division. While the obedient and loyal Izuru and Momo became the lieutenants of the 3rd and 5th Divisions under Gin and Aizen, respectively, Renji proved to be too rebellious and was transferred to the 11th Division, later transferring to the 6th Division to become its lieutenant.
Gin and Tōsen later became captains of the 3rd and 9th Divisions, respectively, but remained loyal only to Aizen. According to Aizen, even when Momo was his lieutenant, he always thought of Gin as his subordinate.
At some point during their captaincy, Aizen, Gin, and Tōsen secretly visited Baraggan Louisenbairn, King of Hueco Mundo and Lord of Las Noches. He and Baraggan engaged in a light conversation, with Aizen asking Baraggan to look at his Zanpakutō. When Aizen asked Baraggan if he was happy in his position and if he would join him in a new world where he would be granted more power, Baraggan dismissed this notion and stated there was no power higher than his. Releasing his Shikai, Aizen showed Baraggan the decimation of his army, thus subjugating him in turn. This created great enmity between Aizen and Baraggan, who promised to seek Aizen's destruction for all eternity.
Sometime after the Hollowfication Incident and during his and Isshin Shiba's time as captains, he, Gin, and Tōsen were still working on their Hollowfication experiments and attempting to locate Shinji's group. He told his co-conspirators to remain patient and reassured them everything would go as planned.
Soon after this, Aizen and his two comrades watched the fight between Isshin and one of their Hollow creations on a monitor, with Aizen wondering why Isshin was present in Naruki City. When Tōsen stated Isshin did not report that he was going and Gin said this could cause them immense trouble, Aizen, intrigued by this interesting turn of events, referred to Isshin's arrival as an unexpected harvest and asked if it was possible for him to watch the battle at a closer distance. Later, when he arrives at the scene of the battle with Gin and Tōsen, the three of them wearing Reiatsu-concealing cloaks, Aizen cut down Isshin from behind. Commenting on the quality of Urahara's cloak and how simply kneading Kidō into it will let the wearer blend into the air, Aizen prepared to leave, stating that Isshin would not be able to use Bankai in his current state and that the Hollow experiment was a failure if a captain could take it out using only their Shikai. After Tōsen expressed his desire to call the sample "White" because this was their first experiment based on a Shinigami's soul, Aizen said it was ironic to call the Hollow "White" because of its inner whiteness while its outside was completely encased in black. After observing the fight for a time, Aizen and his cohorts noticed the involvement of a Quincy in the fight, which surprised the three of them. When Tōsen decided to eliminate her, Aizen stopped him from killing her and eagerly stated they should see what happens next.
Following White's defeat and destruction, Tōsen expressed his shock and frustration at the events, calling it a failure. Calming him down, Aizen said that the results were interesting and expressed curiosity on how events would further unfold.
Aizen became aware of the existence of Ichigo Kurosaki when he was born. Understanding the significance of his birth, Aizen decided to aid Ichigo's development for his plans. To this end, he had Rukia Kuchiki stationed in Karakura Town. Aizen also put into place obstacles which would force Ichigo to grow stronger, including the Hollow which attacked his family and the Menos Grande which appeared when Uryū Ishida used the Hollow bait.
Soul Society arc
When Rukia Kuchiki is brought back to Soul Society with the Hōgyoku concealed within her body, Aizen manipulates everyone into an elaborate conspiracy by killing all the members of Central 46 and beginning to move Rukia's execution date up. He begins to plant suspicion within Soul Society, initially speaking of the possibility of a conspiracy with 6th Division Lieutenant Renji Abarai, and tries to use Gin Ichimaru's intrusion in repelling Ichigo Kurosaki and the Ryoka to orchestrate suspicion among the gathered captains, particularly 10th Division Captain Tōshirō Hitsugaya, who witnesses a conversation between the two personally. Later, Aizen uses his Zanpakutō's ability to trick everyone into believing he has been killed, and has Gin lead everyone to believe that Gin himself is the murderer.
Aizen sets up his lieutenant, Momo Hinamori, to believe he was killed by Hitsugaya, her childhood friend, to get her out of the way. Because of the nature of Rukia's sentence and execution method, the Gotei 13 splits into factions and the captains begin to fight among themselves. During the turmoil, Gin executes the overt aspects of the plan, which begins to be revealed when 4th Division Captain Retsu Unohana begins to suspect there is something wrong with Aizen's supposedly dead body.
Aizen reveals himself by nearly killing his lieutenant and defeating Hitsugaya. Commenting on how nice it is to see ice at this time of year, Aizen is stopped by Unohana, who has figured out something is wrong with his corpse doll, only for Aizen to bring out his corpse doll and disclose to Unohana and her lieutenant, Isane Kotetsu, the true abilities of his Zanpakutō. This allows Unohana to deduce the ritual for being affected by the hypnosis, after which Aizen reveals Kaname Tōsen has always been his subordinate because he cannot see. Aizen has Gin bring the two of them to Sōkyoku Hill, where Tōsen had brought Renji and Rukia to him.
When Aizen asks Renji to put Rukia down, Renji refuses. Commenting on Renji's stubbornness, Aizen says that it is acceptable if Renji wishes to hold on to Rukia, but he will have to leave the arms he is holding her with behind. Drawing his sword, Aizen attacks Renji, but Renji is able to draw his sword in time to dodge Aizen's attack, leaving only his right arm damaged. Applauding his ability to dodge, Aizen expresses happiness with the improvements in Renji's skill because he was one of Aizen's subordinates. Telling Renji he does not want things to be harder because while he finds it difficult to hold back his strength, Aizen states he finds it unbearable for a subordinate to die.
Challenging this assertion, Renji asks Aizen why he killed Momo. When Aizen states it was an act of compassion because he has trained Momo so she could not live without him, Renji says Aizen is not the captain he knew, prompting Aizen to state he was the same man, but neither Renji nor anyone else had realized what he was. When Renji attacks Aizen with his Shikai, Aizen blocks it with his bare hand and easily cuts the segments of the blade apart while also cutting up Renji. When Renji refuses to give up Rukia, Aizen prepares to kill him, only to be stopped by Ichigo.
Ichigo teams up with Renji, but even after using Renji's strongest attack with his Shikai to create an opening for Ichigo, the attack fails. Displaying his power by blocking Ichigo's blade with a single finger and nearly cutting him in half, Aizen cuts down Renji and approaches Rukia. Beginning to drag Rukia off, Aizen explains his plans to the critically injured but still conscious Ichigo, only for 7th Division Captain Sajin Komamura to appear and attack him. Critically injuring Komamura with Hadō #90. Kurohitsugi, Aizen removes the Hōgyoku from Rukia's body using information obtained from Kisuke Urahara's research hidden in the chambers of Central 46. Surprised the procedure did not kill Rukia and having no further use for her, Aizen orders Gin to kill Rukia as he prepares to leave, only for 6th Division Captain Byakuya Kuchiki to save Rukia by taking the blow himself.
Turning around, Aizen prepares to draw his Zanpakutō in order to finish Rukia himself, but is stopped by Yoruichi Shihōin. When he is threatened with death from 2nd Division Captain Suì-Fēng's Zanpakutō if he moves, Aizen calls Yoruichi a nostalgic face and notes how not even Yoruichi and Suì-Fēng will be able to come after him while fighting the hypnotized Gatekeepers. As Kūkaku Shiba and Jidanbō Ikkanzaka arrive to assist them, Gin is restrained by 10th Division Lieutenant Rangiku Matsumoto and apologizes for being unable to help Aizen. As they are surrounded by every captain and lieutenant of the Gotei 13 still capable of moving, Aizen smiles and says it is time. Realizing something is wrong, Yoruichi runs away and tells Suì-Fēng to get away. When Aizen, Gin, and Tōsen are bathed in light, Captain-Commander Genryūsai Shigekuni Yamamoto calls the others off and reveals Aizen, Tōsen, and Gin became completely untouchable the moment they were covered in this light, called a Negación, a separate world created by Menos Grande when rescuing one of their own which transports them to Hueco Mundo.
While leaving, Aizen removes his glasses and changes his hairstyle by combing a hand through it. As his glasses evaporate into dust, Aizen announces his goal to stand atop of the world in the heavens and take the place of the absent gods, something no one has ever done before. Before finally ascending into the portal created by the Menos' arrival, he bids farewell to all of the Shinigami present and tells Ichigo he was very interesting for a Human.
From there, Aizen changes his clothing style to resemble the Arrancar, and begins to create several Arrancar. At some point, he finds Coyote Starrk and Lilynette Gingerbuck and is impressed by the mountain of dead Hollows nearby. When Aizen asks Starrk if the Hollows were killed by him, Starrk tells him the Hollows just died. When Aizen makes an offer to Starrk to join him as an ally, Starrk believes Aizen to be strong and agrees to join him. Later, Aizen sends Grand Fisher and two other powerful Hollows to the Human World. Grand Fisher is killed by Isshin Kurosaki, and the other two Hollows are killed by Ryūken Ishida. Isshin and Urahara theorize Aizen sent these pseudo-Arrancar to the Human World to look for information on Ichigo.
Later, after examining Orihime's powers, Aizen decides to initiate another invasion on Karakura Town, letting Ulquiorra pick the members of the invasion force, though Aizen personally sends Grimmjow as well. Ulquiorra, Grimmjow, Yammy, Luppi Antenor, and Wonderweiss Margela go to invade Karakura Town to capture Orihime. Ulquiorra succeeds in forcing Orihime to join Aizen's side and brings her to Las Noches, where he has her display her power by restoring Grimmjow's amputated left arm and reveals to the Arrancar present how Orihime's powers are rejection, meaning she can "reject" anything which has happened.
Hueco Mundo arc
Calling the Espada to a meeting, Aizen invites them to have some tea. As they are drinking their tea, Aizen reveals to them the presence of Ichigo, Uryū, and Sado. When the Espada begin to dismiss the group as not dangerous, Aizen warns them to not underestimate the trio and reveals their exploits in Soul Society. When Aizen identifies Orihime as the missing member of their group and states they have come to rescue her, Grimmjow begins to leave and states he is going to go eradicate the threat in Aizen's name. Stopping him, Aizen resorts to intimidating Grimmjow with his Reiatsu when the latter hesitates to back down. As Grimmjow is brought to his knees by Aizen's immense Reiatsu, Aizen tells the other Espada to return to their palaces and wait patiently. Telling them not to be overconfident, Aizen says they do not to be afraid because no one can defeat them as long as they are with him.
At one point, Aizen has Orihime brought to his room by Loly Aivirrne and Menoly Mallia. After sending the two Arrancar away, Aizen shows Orihime the Hōgyoku. Asking Orihime to interpret him showing her the Hōgyoku as a sign of his trust in her, Aizen asks Orihime to use Sōten Kisshun to undo some minor damage which has occurred to the Hōgyoku from being repeatedly reawakened.
Following the arrival of Ichigo and company at Las Noches, Aizen is informed of Dordoni Alessandro Del Socaccio's death by a messenger. Unsurprised by Dordoni's demise, Aizen expresses surprise at Ichigo struggling against him. Cornering the messenger by catching him in a lie of omission, Aizen demands to know who sent the Exequias after Dordoni. When the messenger does not answer, Szayelaporro Granz intervenes and reveals he is the one who did so before begging Aizen's forgiveness, which Aizen grants him. As Szayelaporro turns to leave, Aizen tells him to make his reports more thorough next time, shocking Szayelaporro, before asking him if the Reiatsu record he obtained from Dordoni will aid in his research. Confirming this, Szayelaporro leaves. Following his departure, Gin, who had been present in with Aizen the entire time, asks Aizen if he is enjoying himself even though his subordinates are losing, prompting Aizen to confess the emotion he is feeling at the moment is akin to enjoyment.
After Aaroniero Arruruerie, Szayelaporro, Zommari Rureaux, Grimmjow, and Nnoitra Gilga are defeated by Ichigo, his friends, and the invading Shinigami captains, Aizen has Coyote Starrk bring Orihime to him. Telling Orihime to smile and wait where she is while they go and destroy Karakura Town, Aizen has Tōsen use Bakudō #77. Tenteikūra. Using this, Aizen reveals his true motivation behind the capture of Orihime to Ichigo and the others: by kidnapping Orihime, he inspired Ichigo and his friends to attempt a rescue, thereby depriving Soul Society of a valuable military asset, and when four captains assisted in her rescue, adding to the three which had already defected, Soul Society was left at half its regular strength, so he and his forces are now ready to attack. Leaving with Gin, Tōsen, Tier Harribel, Baraggan Louisenbairn, Starrk, and their Fracción to invade Karakura Town, Aizen leaves Las Noches in the hands of Ulquiorra.
Fake Karakura Town arc
Leaving to destroy Karakura Town, Aizen seals the portals Ichigo and the others used to access Hueco Mundo, effectively stranding them there until his return, and leaves Ulquiorra in charge of Las Noches. When Soul Society substitutes the town for an abandoned duplicate, Aizen muses on how he will have to travel to Soul Society to destroy the real town after he and his forces defeat the remaining captains. Before his group is able to take action, Aizen is trapped within Yamamoto's Jōkaku Enjō with Gin and Tōsen and is unable to fight. Claiming such action would be unnecessary, Aizen states the power of his Espada alone will be enough to defeat the Gotei 13.
When Momo arrives in the fake Karakura Town to assist Rangiku Matsumoto, Aizen has a somewhat intrigued look in his eyes upon sensing his former lieutenant, whom he assumed he killed, is alive. When Gin asks him if anything is wrong, Aizen says Momo's presence will not make any difference.
Arriving in the fake Karakura Town alongside Wonderweiss, Hooleer extinguishes Jōkaku Enjō with its breath. Stating the smell of death is splendid, Aizen walks free with his two comrades and says the smell fits the occasion. As everyone is alerted to Aizen's presence, 3rd Division Lieutenant Izuru Kira fears Soul Society may have lost, only for the Visored to arrive. After a short battle against a group of Gillians, Shinji Hirako confronts Aizen. Attempting to slash at Aizen, Shinji is stopped by Tōsen as Aizen looks on.
As the battle continues, Baraggan comes close to his death and throws his blade at Aizen in a desperate attempt to kill the person responsible for Baraggan losing his position as King of Hueco Mundo, only for Baraggan's Respira to destroy the axe before it can even reach Aizen, who looks away. As both Starrk and Harribel notice this, Starrk notes Aizen has nothing to say at Baraggan's passing.
Observing Starrk's defeat as he falls to the ground, Aizen subsequently takes notice of the fight between Tier Harribel and Hiyori Sarugaki, Lisa Yadōmaru, and Tōshirō Hitsugaya. Stopping the fight between Gin and Shinji, Aizen tells Gin this is enough and states it is time to end this fight. Using Shunpo, Aizen appears near Harribel's battle and slashes her straight across her torso with his Zanpakutō, seriously wounding her. As Harribel expresses surprise, Aizen tells her it appears she is not strong enough to fight for him before telling Gin and Tōsen it is time to go.
As Aizen laments to Harribel how he alone is more powerful than all of the Espada he went through so much trouble to gather, an enraged Harribel impales him with her sword. As Aizen states dealing with her is such a pain, a surprised Harribel realizes she attacked an illusion and has been stabbed in her left shoulder by Aizen. Telling Harribel he will never let her raise her sword to him again, Aizen withdraws his sword. As Harribel falls to the city below, Aizen tells everyone to come at him.
When Shinji warns the other Visored to not approach Aizen too quickly, Aizen says they already died 100 years ago, so they have nothing to fear. Charging Aizen, an enraged Hiyori is bifurcated by Gin.
Going to Hiyori's side, Shinji angrily stares down Aizen, who states there is finally life in Shinji's eyes. As Aizen says he will unsheathe his sword especially for him, Shinji approaches his former lieutenant to engage him in battle. When Shinji tells the others to wait for Ichigo to arrive, Aizen asks Shinji why he would put his faith in Ichigo. When Shinji tells Aizen he does not expect him to understand because he does not even trust his own followers, Aizen says only the weak rely on others. When Shinji questions how he can say this with all the followers he has, for he is sure Aizen has asked them to trust him, Aizen states he is wrong because he never asked them to trust him at all. Revealing he told them to come with him, but he has never told them to trust him and has told them to not trust anyone, including himself, Aizen laments there are not many strong enough to do that. Aizen further explains how everyone wishes to believe in something bigger than themselves, and he will become the god everyone is forced to put their faith in.
Commenting on Aizen drawing his blade, Shinji asks Aizen if he is scared. When Aizen remains silent, Shinji says there is no point in ignoring him because he still has to be uneasy no matter how strong he is. Quoting Aizen's own words from 100 years ago about how Shinji had never opened up to him, Shinji tells Aizen he does not have a clue what kind of powers his Zanpakutō has because of this. Telling Aizen he is wrong if he believes Kyōka Suigetsu is the only Zanpakutō with the ability to control other people's senses, Shinji releases his Zanpakutō, Sakanade.
The release of Shinji's Shikai prompts Aizen to note it is an interesting shape for a katana. As the ability of Sakanade takes effect and reverses the world around them, Shinji attacks and manages to cut Aizen. After another attempt, Aizen figures out the ability and cuts Shinji before stating the ability is child's play. Moments later, he looks down as Komamura yells up at him. As the sky begins to crack, Ichigo breaks through behind Aizen.
As Ichigo's Getsuga Tenshō hits a barrier which Aizen had created, Aizen taunts Ichigo, who attacks again. Dodging, Aizen continues his taunts. When Ichigo begins to waver, Komamura appears beside him and tells Ichigo not to be disheartened, for provoking his opponents is Aizen's forte. He and the other Shinigami and Visored prepare to help Ichigo fight Aizen while avoiding witnessing the release of his Shikai. When Hitsugaya starts the attack, Aizen mocks this, prompting Hitsugaya to state someone needed to start. Becoming angry, Hitsugaya releases his Bankai. Attacked by Hitsugaya, Aizen dodges every attack from Hitsugaya.
As Aizen prepares to counterattack, Komamura and Love Aikawa attack him, but Aizen stops them. When Komamura activates his Bankai, Kokujō Tengen Myō'ō, Aizen slices through it, cutting off Komamura's arm. As Aizen attacks Komamura once more, Rōjūrō Ōtoribashi tries to attack Aizen, only for Aizen to pull Rose towards him and slash him, sending Rose falling to the ground. Defeating the remaining Visored, Aizen calls them "Pseudo Arrancar".
Moving in front of Aizen, Suì-Fēng mocks him and creates several clones of herself with Shunpo. As the clones attack, Hitsugaya encases Aizen in ice as 8th Division Captain Shunsui Kyōraku emerges from a shadow to attack him. As Hitsugaya attacks him, Aizen glances at him and notes Hitsugaya does not plan ahead due to his young age. Noticing Shinji holding Sakanade, Aizen realizes it is too late to react because he is in the inverted world as Hitsugaya impales him.
Being the only one who has not yet seen Aizen's Shikai, Ichigo shouts at the captains, causing them to realize Aizen used Kyōka Suigetsu and switched places with Momo. Having been lying down next to Izuru and 7th Division Lieutenant Tetsuzaemon Iba, Aizen gets up and walks away after his illusion is revealed. Believing it is Momo who is walking away, Izuru and Iba try to follow him, but are cut down. When Shinji angrily asks Aizen when he cast the illusion, Aizen asks them when they began thinking he was not using Kyōka Suigetsu. Overcome with anger at being fooled by Aizen into attacking Momo, Hitsugaya charges at Aizen while Shunsui warns him to wait. Telling them they are all completely open, Aizen cuts down Hitsugaya, Shunsui, Suì-Fēng, and Shinji.
As Aizen defeats the last of his attackers, Yamamoto draws his attention with a huge pillar of fire. Stating Yamamoto is too late and the Gotei 13 will crumble with his defeat, Aizen taunts Yamamoto before stabbing him. Pulling Aizen in, Yamamoto says he can tell it is not an illusion. Activating Ennetsu Jigoku, Yamamoto states everything caught within it will burn. Appearing behind Yamamoto in his Resurrección, Extinguir, Wonderweiss sends Yamamoto crashing to the ground below as Aizen reveals Wonderweiss was made for the sole purpose of stopping Yamamoto's abilities. Employing his hand-to-hand combat skills, Yamamoto defeats Wonderweiss. When Aizen says Yamamoto's flames were all stored within Wonderweiss, Yamamoto attempts to contain them as Wonderweiss' body explodes. Afterwards, as Aizen approaches, a battered Yamamoto uses Hadō #96. Ittō Kasō.
Pouncing on Aizen, Ichigo sends him crashing through a nearby building. As the wound he sustained from the attack closes up, Aizen reveals he has placed the Hōgyoku, which instinctively heals its master, in his chest. Complimenting Ichigo's Reiatsu, Aizen states Ichigo has grown as he had planned, surprising Ichigo. Listing various important occurrences in Ichigo's development, Aizen claims they all happened as he desired. When Aizen explains how he found Ichigo to be the perfect subject for his research, Ichigo says Aizen must be lying because of what he said in Soul Society. Revealing he was lying back then, Aizen begins to say why he thought Ichigo would be perfect, but is interrupted by the arrival of Isshin, who states Aizen has said too much. When Isshin moves Ichigo away, Aizen notes this was a smart move. When Aizen asks Gin how long he has been watching, Gin states he did not see an opportunity to help. Attacking Aizen, Isshin sends him crashing through multiple buildings. As their battle continues, Isshin notes Aizen is tiring, prompting Aizen to reveal he has reached the limit of his Shinigami self and the Hōgyoku has begun to understand his desires.
Saying the Hōgyoku possesses a will of its own and he only became aware of this after becoming its master, Aizen reveals the true power of the Hōgyoku is not removing of the boundaries between Shinigami and Hollow, but capturing and materializing the hearts around it. Claiming it has brought about the events surrounding Ichigo, Rukia, and Urahara, Aizen states Urahara misunderstood the nature of the Hōgyoku because the breakdown of the barrier between Shinigami and Hollow was his own desire. Claiming Ichigo, Sado, and Orihime all got their powers from the Hōgyoku, Aizen explains more about it before being interrupted by Gin knocking Ichigo into a nearby building. As Ichigo talks to Isshin, Gin asks Aizen if he interrupted his speech. As Aizen says he had reached the end of his speech, his body is enveloped by the Hōgyoku. Stating the Hōgyoku deserves its name, Aizen is interrupted when he is shot from behind by Urahara, whom he welcomes onto the battlefield.
After Urahara says it has been a long time since last they met, the two banter about Aizen's fusion with the Hōgyoku. Stating it is not fusion, Aizen claims he has subjugated the Hōgyoku, which Urahara had failed to master. Glaring down at Aizen, Urahara admits it is true he was unable to master it in the past. Questioning his statement, Aizen says he must be unable to admit defeat. Retracting the question, Aizen impales Urahara and tells him he has already lost the chance to command the Hōgyoku. As Urahara's body bursts, Urahara appears and uses Bakudō #61. Rikujōkōrō. Revealing he saw the portable Gigai trick in Yammy's report, Aizen admits he let his guard down, for he never thought Urahara would use such a petty trick, and asks Urahara what he will do after constricting him. Asking Aizen why he believes Urahara would only use the level of Bakudō, Urahara uses Bakudō #63. Sajō Sabaku and Bakudō #73. Kuyō Shibari. When Urahara begins chanting an incantation, Aizen asks him if he believes Aizen will allow him to use this Kidō. Stating it is too late to stop it, Urahara fires Hadō #91. Senjū Kōten Taihō. After the smoke explosion dissipates, Urahara notes Aizen has let down his guard because of his new found power. Confirming this, Aizen appears behind Urahara and attacks him. When Aizen notes the Hōgyoku has made him invincible and he has no need to dodge a level 90 Kidō, Urahara says he was talking about how Aizen would never have come into physical contact with him twice without a plan. Trapped by spiritual energy cuffs at his wrists, Aizen stares in shock. As Urahara explains the nature of the seal, Aizen is enveloped in a pillar of light.
As the smoke clears, Aizen appears behind Urahara in a new form and states this would have worked if he had not subjugated the Hōgyoku. As Aizen says the Hōgyoku is beyond Urahara's comprehension and this battle is a waste, Urahara unsheathes his Zanpakutō. As Isshin comes up from behind, he and Urahara restrict Aizen with chains. Expressing confusion, Aizen demands to know what is going on as Yoruichi Shihōin attacks him from above. Smashing into his back with great force, Yoruichi pummels Aizen into the ground. As Urahara tells Yoruichi to move away, Aizen recovers and destroys the armor on Yoruichi's left leg. Emerging from the rubble with his new form severely cracked, Aizen proclaims he will destroy all of their attacks. noting Yoruichi's armor must be special for her to survive his attack, Aizen states he plans on destroying this.
Having shattered more of Yoruichi's armor, Aizen watches as Yoruichi and Urahara bicker again. Asking Aizen why he is staring at him, Urahara says Aizen is showing interest again. When Aizen reveals he finds Urahara interesting because he is the only one in Soul Society who exceeds his intellect, Urahara attacks Aizen, but Aizen blocks it and Yoruichi's sneak attack. As Aizen is about to counter her, Yoruichi activates Shunkō and knocks Aizen to the ground, but her next attack is blocked. Intervening, Urahara binds him with Shibari, Benihime and uses Hiasobi, Benihime, Juzutsunagi, creating a massive explosion. Afterwards, Isshin attacks Aizen with a Getsuga Tenshō. Emerging from Isshin's attack with a large crack down his new form's face, Aizen defeats Isshin, Urahara, and Yoruichi, and gathers Gin as he heads off towards Soul Society.
Arriving at the Dangai with Gin, Aizen notices the approaching Kōtotsu. Destroying it, Aizen asks Gin what he was afraid of and states reason only exists for those who cannot live without it. As the two reach the Dangai's exit, Aizen declares they will go to the edge of reason. As they arrive, Aizen notices they are not in Karakura Town because he destroyed the Kōtotsu. As they walk, they come into contact with a man who turns to dust upon getting close to Aizen due to his spiritual power. When Gin spots Tatsuki Arisawa, Aizen identifies her as Ichigo's ally. When Aizen and Gin approach, Tatsuki and Keigo Asano are able to withstand his power. When Keigo runs, Aizen says they will start with Tatsuki.
When Don Kanonji hits him with a blast of energy, Aizen emerges from the smoke unscathed. As Kanonji reaches to hit Aizen, Rangiku arrives. Rushing in, Gin says he will take care of her and takes Rangiku away as Aizen says Gin is an interesting child. Catching up to Tatsuki, Keigo, and Mizuiro Kojima, Aizen is about to kill them when Zennosuke Kurumadani shows up and uses his Shikai's special ability to trap Aizen under rubble, though he breaks free. As the Humans and Zennosuke run away, Gin returns, prompting Aizen to ask him what happened to Rangiku. When Gin claims he killed Rangiku, Aizen admits he is surprised because he thought Gin had feelings for her. Likening himself to a snake, Gin says he has no feelings.
When Gin asks Aizen about his plans, Aizen reveals them. Stating this is good, Gin places his hand on the blade of Kyōka Suigetsu. Throwing up his sleeve, Gin releases Kamishini no Yari from under his arm and impales a surprised Aizen through the chest. As Gin reveals the one weakness in Aizen's Zanpakutō, a bleeding Aizen tells Gin he knew Gin would betray him. Revealing the true ability of his Bankai, Gin places his hand up to Aizen's chest and activates Korose, Kamishini no Yari. As Aizen curses him, Gin watches as Aizen's chest begins to dissolve from the inside out, leaving a gaping hole with the Hōgyoku floating in the middle.
As Aizen's body continues to erode, Gin grabs the Hōgyoku, but Aizen throws Gin away. Gin runs off with the Hōgyoku, leaving a slowly dissolving Aizen to fall to the ground. Uttering Gin's name, Aizen yells in rage as a pillar of light surrounds him. As the pillar of light splits to reveal Aizen's new winged form, the Hōgyoku in Gin's hand disappears and rejoins Aizen, who tells Gin the Hōgyoku is under his control whether it is in his body or not. Appearing behind Gin, Aizen cuts him deeply with one swing of his sword. Recalling his failure with his own Hōgyoku, Aizen thanks Gin and throws him through a building. Arriving at the scene, Rangiku goes to Gin's side. Closing in on them, Aizen is interrupted by the sudden arrival of Ichigo. As Aizen says he is disappointed Ichigo has not evolved, Ichigo states he wishes to move their fight away from his friends. Thinking Ichigo cannot harm him, Aizen refuses. Grabbing Aizen, Ichigo brings him outside Karakura Town. Throwing him into the ground, Ichigo tells a stunned Aizen he will finish the fight in an instant.
Commenting upon Ichigo's words, Aizen theorizes about the nature of his power. As he and Ichigo clash, leveling a nearby mountain, Aizen notes he and Ichigo have both reached the peak of their evolutions. Noting he understands the dimensions of his Ichigo's evolution and his own are very different, Aizen states he can bring himself to crush Ichigo's sword with a single blow. When Aizen attacks, Ichigo stops it with his bare hand. Expressing surprise, Aizen tells Ichigo to not get so excited. Proclaiming he will crush Ichigo with Kidō, Aizen uses a full incantation of Hadō #90. Kurohitsugi. As Aizen gloats at the full power of his Kidō, Ichigo destroys it and tells Aizen he is stronger as he attacks.
As Aizen moves away in a flash of light, Ichigo taunts him. When the diamond on Aizen's head opens up, he transforms once more. Firing an energy blast which damages Ichigo's left arm, Aizen uses a circular blast which surrounds his body, but Ichigo destroys it with a wave of his sword. As a shocked Aizen looks on, Ichigo tells him he will end the fight with Saigo no Getsuga Tenshō. As Aizen looks on in wonder at the new form Ichigo is taking, Ichigo explains what Saigo no Getsuga Tenshō is and its costs. Amazed at how he still feels absolutely nothing from Ichigo despite this new transformation, Aizen theorizes Ichigo may have surpassed him. As an enraged Aizen proclaims such a thing is impossible, Ichigo uses Mugetsu, enveloping the battlefield in darkness.
Ichigo's attack envelops Aizen and severely wounds him. After Aizen crashes into the ground, the Hōgyoku regenerates his wounds. As Ichigo moves to finish Aizen, his powers begin to fade. When Kyōka Suigetsu begins to fade as well, Aizen tries to finish off Ichigo, but an energy spike emerges from his chest before he can do so. Arriving, Urahara takes responsibility for the attack and reveals he fired a Kidō into Aizen's body on the back of another Kidō before Aizen achieved complete transformation. When Urahara explains it is a seal because he believed Aizen would not be able to be killed, more spikes of energy emerge from Aizen's chest. Proclaiming he hates Urahara, Aizen demands to know why someone of Urahara's intellect allows himself to be controlled by "that thing". Realizing Aizen is referring to the Soul King, Urahara notes Aizen has seen it and tells Aizen Soul Society would be rent asunder without the Soul King's existence. Proclaiming victors must always speak of the way the world should be and not the way it is, Aizen is sealed within Urahara's Kidō.
Ten days later, Aizen sits bound to a chair by many black bands, where he is standing trial under the authority of the judges of the Central 46 for high treason and a list of many other crimes. When they sentence him to 18,800 years in Muken, the 8th and lowest level of the underground prison, Aizen states it is comical to receive judgment from the likes of them. Silencing him, a judge calls him a traitor and warns him to not get cocky just because he is immortal. Ordering Aizen's eye and mouth bound at once, the judge raises his sentence up to 20,000 years.
The Thousand-Year Blood War arc
During the Wandenreich's invasion of Soul Society, Yhwach reveals Aizen was being held underneath the First Division, where he offered him a position in their army as a Special War Power, only to be refused by the prisoner. Subsequently, Yhwach discovers he lost track of time during his meeting with Aizen.
Hōgyoku (崩玉, Crumbling Orb; Viz "Breakdown Sphere"): Aizen has stated he arrived at the conclusion of the Hōgyoku before Kisuke Urahara, though it ended in failure. In order to finalize the Hōgyoku, the souls of hundreds upon hundreds of Shinigami and hundreds and hundreds of Rukongai citizens who bore Shinigami potential needed to be sacrificed to it, but it still was not satisfied. According to the experimental data, Urahara's Hōgyoku was also incomplete, so Aizen decided to steal this Hōgyoku and present it to his own. Produced as a result of Aizen's Hollowfication research, this Hōgyoku is what gave the Visored their Hollow powers. This process seems to spread as some form of infection through wounds, which only has the desired effects on souls with high amounts of Reiryoku and not souls with low amounts of Reiryoku. Those infected experience great sickness and fatigue until they vomit up a white liquid, which forms the beginnings of their Hollow mask before proceeding to transform their bodies and warp their minds. Other than this, all which is known is the process is accelerated by aggressive emotions. The conclusive result turns a Shinigami into a primarily Hollow-like humanoid, which is a highly unstable transformation causing mindlessness and aggression.
Caja Negación (反膜の匪 (カハ・ネガシオン), kaha negashion; Spanish for "Negation Box", Japanese for "Anti-Membrane of Negation"): This cube traps a subject in an alternate dimension from which there is no escape; however, this assumes it is being used as intended. More powerful Arrancar, namely the Espada themselves, can escape, given enough time.
Hollow Research and Development: During his tenure as a Shinigami and after his defection, Aizen spent much time researching Shinigami and Hollows in order to break down the boundaries between them. Because of this, he was able to create Hollows with Shinigami-like abilities. He created Hollows with the ability to hide their Reiatsu like the ones who attacked Renji Abarai, Izuru Kira, and Momo Hinamori during their training, Hollows, like Metastacia, with the ability to shatter a Shinigami's Zanpakutō and fuse with them, and even a Hollow using a Shinigami's soul as a base. However, according to Aizen, all of these ended in failures. After his defection, he began using the Hōgyoku to create Arrancar who were far more powerful than naturally occurring Arrancar. Aizen gave Hollow powers to a Shinigami, namely Kaname Tōsen, who gained both a Hollow's mask and a Resurrección.
Reiatsu Concealing Cloak: A cloak created by Urahara which hides the wearer's Reiatsu. Aizen used this cloak to sneak up and attack Isshin Shiba. Aizen states it allows the wearer to blend in with the air around them by incorporating Kidō into the cloak, almost completely concealing them from sight.
Powers & Abilities
Genius Intellect: Arguably, the most fearsome trait of Aizen is his tremendous intellect. He is well-versed in the history, strategy, and tactics of Soul Society, and is privy to knowledge unknown to many, such as the existence of the Ōken, as well as the obscure method in which to create one. He has created several Hollows and Arrancar while avoiding arousing any suspicion from the majority of his fellow Shinigami. For over 110 years, he had been engaging in experiments which involved unique and highly dangerous manipulations of Reiryoku and Reishi, including experiments involving the Hollowfication of Shinigami, something which had never been done before. He has an innate knowledge of the Hōgyoku, rivaling that of its creator, Kisuke Urahara; he even knows how to destroy it. He figured out the true nature of Shinji Hirako's Shikai and deduced it produces an optical illusion, allowing him to counter its effect. He effectively deduces the true nature of the Hōgyoku via trial and error of the situations which revolve around it.
Master Manipulator: Aizen has proven himself very crafty and a cunning man since his public betrayal of Soul Society. He can deceive and manipulate others around him in different ways for a variety of purposes. For years, he convinced everyone around him he was a kind-hearted man with the best intentions for Soul Society while performing horrific actions. He has a great talent for analyzing and observing everything in his presence, allowing him to understand a person's pattern of thinking and better manipulate them. He is a master strategist, formulating well thought out plans several steps ahead of others. These plans are incredibly involved and detailed, and are planned out with the long term in mind. His most well-known plan spans over 110 years, a great accomplishment which involved moving people around like pieces on a chessboard and making sure they played the role he had planned out for them. He is masterful when it comes to plans within plans, such as kidnapping Orihime, allowing him to use her abilities to extend the shelf life of the Hōgyoku, and, after the departure of Gin Ichimaru, Kaname Tōsen, and himself, trapping Ichigo Kurosaki, Yasutora Sado, Uryū Ishida, and four captains in Hueco Mundo, making it easier for him to accomplish the goal of his first plan: creating the Ōken and gaining access to the Royal Palace.
Master Strategist & Tactician: Aizen has proven himself to be a masterful and flexible tactician, adjusting seamlessly to any given combat situation, if not planning out events well in advance. He understands the strengths, weaknesses and mannerisms of his enemies, and creates strategies to achieve victory; this includes modifying an Arrancar for the sole purpose of defeating Captain-Commander Genryūsai Shigekuni Yamamoto, a Shinigami whose abilities surpass his own. Despite viewing his power as worlds beyond others, Aizen is highly cautious in battle, placing special protections on himself so as to leave no blind spots.
Shunpo Master: Aizen is so fast, one cannot discern even being attacked by him until after it has taken place. Aizen can easily outmaneuver Ichigo's Bankai and Hollow mask-enhanced speed, to the point where he could casually lay a hand on Ichigo's chest and still dodge a point-blank attack. Aizen can dodge simultaneous attacks at close range and evade Bankai-level techniques. Opponents commonly do not know his movements until after he has made them, and Aizen is skilled enough in his movements to evade at high speeds, even when being attacked from behind.
Master Swordsman: While the full extent of his prowess is unknown, Aizen can fight back against an opponent's Shikai and Bankai with his own blade still sealed. Aizen can perform tremendously powerful and precise strikes and cut through very sturdy targets, such as Renji's Shikai. He cut down four captains (two which used their Bankai) and four Visored (with three being former captains of Gotei 13) with ease.
Hakuda Master: While preferring to finish his fights quickly with his Zanpakutō or Kidō, Aizen is a highly capable unarmed fighter. Repeatedly, Aizen has caught his opponent's weapons with a single bare hand. While fighting offensively, he has shown tremendous agility and dexterity, combining both sword and barehanded skills to simultaneously strike with his sword and kick opponents with great force.
Immense Spiritual Power: Even by captain standards, Aizen possesses a tremendous amount of Reiryoku. 2nd Division Lieutenant Marechiyo Ōmaeda stated Aizen's mere presence is monstrously overwhelming. A hint of Aizen's level of Reiryoku is given when he turns Wonderweiss Margela into an Arrancar before activating the Hōgyoku; Aizen states in spite of its half-awakened state, the Hōgyoku can be fully activated when temporarily fusing with someone with twice as much spiritual power as an average captain-level Shinigami, referring to himself. The force of Aizen's Reiatsu is great enough to, without any visible effort from Aizen, bring Grimmjow Jaegerjaquez, the 6th Espada, to his knees. He has stated it takes a lot for him not to "crush an ant" (referring to Renji) in his path. He claims he is more powerful than all of the Espada. Aizen's Reiatsu is powerful enough to nullify abilities, such as when he negated Suì-Fēng's Nigeki Kessatsu. His Reiatsu is violet.
Immense Strength: Aizen is strong enough to stop the blade of Ichigo's Bankai with just one index finger (albeit Ichigo was already battle-worn from fightingByakuya Kuchiki) and almost cut him in half with a single swing of his sword. He stopped 6th Division Lieutenant Renji Abarai's Shikai with his bare hand and broke it with one swing, injuring Renji in the process. He stopped Komamura's Shikai with his bare hand, proving his strength is more than enough to combat someone of the same level. Aizen further displays his strength by cutting through the sword and wrist of Komamura's Bankai.
Immortality: After being hit by Ichigo's Mugetsu, Aizen somewhat regenerates from the damage, though most of his new form's features have been destroyed and do not return. He is later confirmed by Central 46 to having been left immortal by his interaction with the Hōgyoku.
None. Aizen's Zanpakutō eventually disintegrates, for he claims the Hōgyoku has determined he no longer needs it.
Kyōka Suigetsu (鏡花水月, Mirror Flower, Water Moon): In its sealed form, it resembles a normal katana. The only decoration is the tsuba, which is a hexagon. The two sides parallel to the blade are long, whereas the four sides above and below the blade are shortened, making it into almost a diamond or prism-like shape. It has a green handle.
Shikai: The release command is "Shatter" (砕けろ, kudakero) and is used to both activate and deactivate it.
Shikai Special Ability: The special ability of Kyōka Suigetsu is Kanzen Saimin (完全催眠, Complete Hypnosis). It controls the five senses to the point where it can make the target misinterpret another person's form, shape, mass, feel, and smell to be an enemy's. The initiation condition for the hypnosis is to show the enemy the release of Kyōka Suigetsu. After seeing it just once, this person will completely succumb to the hypnosis. Afterwards, every time it is released, this person will fall under the thrall of the hypnosis. The hypnosis is long lasting; the Visored are still affected by Kyōka Suigetsu even though they were hypnotized at least 110 years ago. Because the spell works when one see the ritual, those who cannot see are consequently immune to the hypnosis. When Aizen turns off the hypnosis by force of will, the image shown melts away or shatters. The complete hypnosis of Kyōka Suigetsu is absolutely flawless; even if the target is aware of being under the hypnosis, they cannot resist its influence. The hypnotic abilities of Kyōka Suigetsu make it a highly useful tool in combat, for Aizen can create simple or complex illusions to deceive his opponents. For example, by creating a decoy of himself, or other illusions to distract his opponents, Aizen can hide his own movements and attack with impunity. His opponent will remain focused on the illusion until it is too late to react. While the illusions Kyōka Suigetsu creates are quite powerful, those with considerable power and skill themselves may be able to note slight differences in the illusions compared to the real thing, even if they are not able to determine what exactly is different. 4th Division Captain Retsu Unohana detected something was amiss with Aizen's fake corpse, even though she did not realize what it was until seeing Aizen alive. The Reiatsu of Kyōka Suigetsu gives it away to those keen enough to sense its actual presence past the illusions created by its complete hypnosis. The sole way to escape the ability of Kyōka Suigetsu is to touch the blade itself before complete hypnosis is activated.
First Fusion (第1の融合): Aizen embeds the Hōgyoku into the center of his chest.
Protective Regeneration: By being embedded into Aizen's chest, the Hōgyoku actively protects Aizen by healing his injuries almost instantly. Aizen withstood a point-blank Hadō #96. Ittō Kasō and only suffered slight burns. Immediately afterwards, he took a point-blank Bankai- and Hollow mask-enhanced Getsuga Tenshō and survived the attack. He emerged unfazed after being hurled into several buildings by Isshin Kurosaki.
Strength Augmentation: Aizen's already considerable strength is further enhanced by the Hōgyoku's evolution of his body. He can strike through a target's flesh with a single, nonchalant sweep of his bare hands.
Durability Augmentation: Aizen's durability is further enhanced by the Hōgyoku's evolution of his body. Because of this, he can withstand Hadō #91. Senjū Kōten Taihō without suffering any damage.
Second Fusion (第2の融合): After Kisuke Urahara attempts to block the Reiatsu vents in Aizen's wrists with a seal, Aizen emerges from the resulting explosion unscathed as a phantom-like figure. Aizen's face is covered with a white canvas, and his outfit is now a white cloak which is split in four sections near his waist. There is a cross in the middle of the cloak, showing where the Hōgyoku is embedded in Aizen. After a short struggle with Yoruichi Shihōin, Aizen's Zanpakutō becomes fused to his arm. When Aizen is attacked, his body appears to crack and fold under damage, only actually bleeding when an attack is powerful enough.
Enhanced Durability: His durability is great enough to block Isshin's Zanpakutō with just his foot and withstand a massive punch from an armored Yoruichi.
Enhanced Endurance: Aizen performs in battle as if he were unharmed, even while taking severe damage. He withstands Isshin's point-blank Getsuga Tenshō and remains unfazed by the attack, despite having a bleeding gash on his forehead.
Enhanced Strength: Aizen created a shockwave after being knocked down by Yoruichi. This shockwave was fast and strong enough to catch even Yoruichi off guard, and subsequently destroyed the Anti-Hierro armor on her left leg.
Enhanced Speed: Aizen's newfound power enhances his speed to the point where even captain-level opponents cannot register his movements. He caught Yoruichi off guard with his speed. He can incapacitate three captain-level opponents seemingly instantaneously.
Regeneration: Aizen demonstrates a form of regeneration, as seen when he recovers from Yoruichi's first attack.
Enhanced Spiritual Power: After Aizen fuses with the Hōgyoku, both Shinigami and Humans are incapable of sensing his Reiatsu unless he voluntarily lowers it and allows them to interfere. He compares this to how a two-dimensional being is incapable of interfering with a three-dimensional one. Isshin surmises Aizen's Reiryoku can only be sensed by someone who is at the same level as him.
Third Fusion (第3の融合): After completing his "chrysalis" stage, Aizen sheds his sleek phantom-like form to reveal a form similar to his original one, with the addition of long hair and gray eyes with purple sclera.
Enhanced Durability: Aizen's durability has been further enhanced. He willingly lets the Kōtotsu collide with him, destroying it, while remaining unharmed.
Enhanced Spiritual Power: Aizen's already enormous Reiryoku is further enhanced. Aizen causes a Human soul to collapse and fade away by standing a few feet away from it. By his own claim, if Don Kanonji had come into contact with Aizen, the former would have ceased to exist. When Aizen touched Gin Ichimaru's arm, his Reiryoku caused the area touched to dissolve. 
Fourth Fusion (第4の融合): After Gin's Korose, Kamishini no Yari creates a hole in Aizen's chest, Reiryoku erupts from his chest, and Aizen emerges from the blast in this new form. In this form, Aizen has a hole in the center of his chest with a cross in the middle of it. The form-fitting white robe which enveloped Aizen in his past two forms lengthens, and flares out at the ends. In this form, Aizen's irises vanish, as well as his eyebrows and the hair curl which usually hangs over his face. There is a diamond-shaped object embedded in his forehead. Aizen has gained three sets of butterfly-like wings. When he faces Ichigo, Aizen's wings become more pointed, rather than rounded.
Regeneration: Aizen can regenerate wounds upon his physical form in a matter of minutes.
Enhanced Strength: In this form, Aizen can attack his opponent with minor movement or force. He tore off Gin's arm and threw him through a nearby building. He caused the ground to tear open with the sheer force of his sword swing, even when the blow was blocked.
Teleportation: In this form, Aizen can de-materialize himself in one place and re-materialize himself a distance away. He breaks down his body into multiple purple shards and reassembles himself.(Unnamed)
Final Fusion (最終融合): After Aizen is struck by Ichigo, the Hōgyoku forces another transformation upon him, which Aizen claims occurs because it does not want him to lose to a Human. In this form, Aizen takes on a distinctively Hollow-like form. The diamond-shaped point breaks open, revealing a vertical black third eye upon the center of Aizen's forehead, and the skin on his face splits open down the middle and pulls back around the sides of his head to reveal a blackened, demon-like skull. Aizen gains three holes running vertically down his chest, the Hōgyoku standing at the center of the top hole. Aizen's feet become single claws, and his hands and feet become blackened, with his right hand fusing with his Zanpakutō. Aizen's butterfly-like wings change into a set of six sheet-like wings, with eyes set upon the center, adorned with Hollow-like skulls, and trailing into tendrils. Aizen no longer has any semblance of clothing in this form, and his musculature becomes more defined.
Fragor (フラゴール, Furagōru; Spanish for "Din/Clamor"): Aizen can release bluish-purple energy spheres from the Hollow-like skulls on the top of his wings. One blast is powerful enough to create a large crater, with an accompanying explosion which creates a shockwave which can be felt miles away from the blast site.
Ultrafragor (ウルトラフラゴール, Urutorafuragōru; Spanish for "Ultra/Beyond-Din/Clamor"): Surrounding his target with his wings, Aizen uses the Hollow-like skulls atop his wings to generate a circle of bluish-purple spiritual energy around a target. Upon creation, the ring reverberates with power and solidifies, while forming three more larger solid rings in concentric formation.
Appearances in Other Media
Aizen is an unlockable character in the Bleach video games. Because he is the central antagonist, most games have Aizen as the final boss, such as in Bleach: Soul Carnival. Hadō #90. Kurohitsugi is one of Aizen's best attacks. In Bleach: Versus Crusade, Bleach: The 3rd Phantom, Bleach: The Blade of Fate and Bleach: Dark Souls, Aizen can use other Kidō spells, such as Hadō #33. Sōkatsui and Bakudō #61. Rikujōkōrō. Aizen makes use of his Zanpakutō's abilities throughout most of the games. Aizen does not appear and is not referenced in any of the films; however, in the second OVA, he drinks coffee at the very end with Gin Ichimaru and Kaname Tōsen. Aizen has his own Bleach Beat Collection, where he sings Shinsen and Kyōka Suigetsu. In Bleach: Soul Resurrección, Aizen appears in his Second, Third, and Final Fusions, but is only playable in his Third Fusion. In his forms, he can use Kidō and is the final boss in the game. Additionally, he is the only character who can move while blocking, though he is also the only character who walks and does not run. In the game, Ultrafragor, upon completion, creates a massive explosion which does heavy damage. Aizen appears in the Rock Musical Bleach shows, where his actor is Kengo Ōkuchi.
IGN.com praises the animators for how Aizen was revealed as the primary antagonist, saying "The Aizen that we first knew was the nicest guy ever, he looked it, and now this Aizen actually looks like the most evil jerk we've ever seen. It's just great how the animators haven't changed his animation much, but have managed to give his face this evil thing about it." However, they were critical of Aizen's convoluted plan, calling it "...so planned out was a bit too planned out to believe. He seemed to know that everything was going to happen exactly as planned..."
Concerning his Zanpakutō, IGN.com described it as overpowered, calling it "the most unbalanced sword in Bleach."  The site found it "far-fetched for some reason to think that nobody can tell when they're getting hypnotized, even the higher ups."
The name of Aizen's Zanpakutō, Kyōka Suigetsu, hints at its illusionary properties; the moon reflected in water and a flower in a mirror are things which can be seen, but not held. It is a Chinese saying for seeing and desiring something which is a dream which cannot be easily grasped in hand, teaching people about temptations.
Aizen's Kyōka Suigetsu came 18th in the Zanpakutō popularity poll. Aizen himself came 23rd in the 4th character popularity poll.
Aizen's theme song, as chosen by Tite Kubo, is "Stand Ablaze" by In Flames.
(To Tōshirō Hitsugaya upon his discovery of an injured Momo Hinamori) "How unfortunate! You found her? Pardon me. It wasn't my intention to traumatize you. Before you could have noticed, I should have chopped her into unrecognizable pieces."
(To Tōshirō Hitsugaya) "You should learn this well, Hitsugaya-kun. Admiration is the emotion furthest from understanding."
(To Renji Abarai) "The Sōsuke Aizen you knew...never existed to begin with."
(To Jūshirō Ukitake) "No one stands on the top of the world. Not you, not me, not even gods. But the unbearable vacancy of the throne in the sky is over. From now on...I will be sitting on it."
(To Shinji Hirako) "You should not dwell on the betrayal that you can see. The truly terrifying betrayal is that which lies utterly hidden from your eyes, Captain Hirako."
(To Tier Harribel indirectly) " I never even dreamed that the power of the ten Espada I went to so much trouble to gather could possibly be inferior to that of myself alone."
(To Shinji Hirako) "To trust in another is the same as to rely upon that person. Reliance is for the weak. It is of no use to us."
(To Shinji Hirako) "It is the nature of all living things to find some being greater than themselves and place their trust in that being, following it blindly. In order to escape from the pressure of that trust, those beings seek a still greater entity in which to believe, and those greater beings too seek still greater, still stronger beings to follow. This is how all kings come to be, and this is how all Gods are born. Do not trust in me yet, Hirako Shinji. I will take my time to teach you the nature of the God whom you face. Then, you shall believe."
(To Ichigo Kurosaki) "Why do you insist on keeping such distance from me? If you wish to make a decisive strike, you should do so at point-blank range. Or perhaps you are afraid of moving in too close, and letting even a part of my body out of your sight? If so, I am disappointed. Keeping one's distance is only a key factor in a battle between two combatants of equal strength. Between you and myself, distances hold no meaning whatsoever. Observe. I am perfectly capable of reaching your heart at any moment."
(To Sajin Komamura) "Did you think that a display of force would be enough to overwhelm me? How foolish. In fact, I would say that the very concept of 'power' itself is different for me than for you. Allow me to demonstrate. This is the true meaning of 'power'."
(To Shunsui Kyōraku, Tōshirō Hitsugaya, Suì-Fēng, and Shinji Hirako) "I will not take your lives. With the measure of power that you possess, you are unlikely even to lose consciousness from those wounds. Stay there, helpless and defeated, and observe carefully as this battle reaches its conclusion."
(To Genryūsai Shigekuni Yamamoto) "Your Ryūjin Jakka is indeed the most powerful Zanpakutō in existence. There is no doubt about that. Were I to fight against it directly, it would likely overpower even me. However, if one abandons all other abilities and focuses his strength entirely on a single specialized skill, then he can overcome even the greatest of powers."
(To Genryūsai Shigekuni Yamamoto) "Cruel? Once a soul has become a Hollow, it holds no meaning. It is a being with no purpose beyond idly consuming its fellow souls. Where do you see cruelty in my giving purpose to such a being? Surely the true act of cruelty was your tearing that soul to pieces."
(To Genryūsai Shigekuni Yamamoto) "I will not do you the injustice of refusing to take your life. You, who are the very embodiment of the history of Soul Society itself - you alone will die here by my blade."
(To Ichigo Kurosaki) "I suppose it is only to be expected. You have my pity. There is no such thing as 'truth' or 'lies' in this world; there never has been. There is only plain, hard facts. And yet, all beings who exist in this world take only those 'facts' that are convenient to them, and take them to be the 'truth'. They do so because they know no other way to live. However, for those powerless beings that make up the majority of this world it is those 'facts' that are inconvenient for their own self-affirmation that make up the real 'truth'."
(To Isshin Kurosaki) "The Hōgyoku is able to manifest the desires of those around it. However, this manifestation will not be successful unless the possessors of those desires have the strength to carry it out. In a way, its ability might be better termed a 'guiding force'. However, it is a peculiar quality of living beings that when it comes to achieving those petty desires that their crude hearts enshrine, they tend to have remarkable levels of success."
(To Kisuke Urahara) "Why not be careless? I no longer have any need to take care. I can feel it. Now that my body has subjugated the Hōgyoku, it has begun to far surpass even the capacity that I once possessed - a capacity that already held no equal in Soul Society. There is no longer any need for me to evade even a Kidō ranking in the nineties!"
(To Gin Ichimaru) "The laws of nature are for beings who must live their lives bound by them. Now, shall we carry on? To a place beyond all laws."
(To Gin Ichimaru) "Fear is necessary to achieve evolution. The fear that, in one's current state, one could quite possibly be destroyed at any time. I thank you, Gin. It is thanks to your presence that I have finally succeeded in surpassing both Shinigami and Hollows."
(To Kisuke Urahara) "Those are the words of a defeated man!! A victor must speak not of the current state of the world, but of how it should rightfully be!"
↑ 1.01.11.21.3Bleach Official Character Book Souls, page 130