One of the saddest deaths in Bleach is that of Gin Ichimaru. He sacrificed most of his life, just so he can learn Aizen’s weakness, and use it against him. But the moment he does so, it’s too late because the Hogyoku has already become so infused with Aizen that he managed to survive Shinzo’s poison even at the brink of death. Aizen then proceeds to cut him down. And as Gin lies there dying beside Matsumoto, fans felt sorry that no one ever knew of this heroism. But isn’t there really?
What if Yamamoto knew that Gin was being a double agent? Back when Rukia and Renji disobeyed their orders and headed to Hueco Mundo to help out Ichigo rescue Orihime, the Gotei 13 sent Kenpachi, Mayuri and Byakuya as back up. What if during that time, Yamamoto was receiving anonymous tips on who most likely would accompany Aizen?
This theory would explain why every captain had somewhat good match-ups with their Espada counterparts: Shunsui with Starkk, Toshiro with Harribel, etc. Imagine if during his stay in Hueco Mundo, Gin was leaking out the abilities of the Espada, and bits of information about Aizen’s plans, to Yamamoto. Of course he wouldn’t be able to say all of it, seeing how sneaky Aizen is, even to his own subordinates. But every bit of detail that Gin can give will surely help.
What makes this theory more impactful is how it affected the final arc. Remember that for a couple chapters in the manga, the Stern Ritters had medallions that can literally steal Bankai. And seeing that Yamamoto had fought against the Quincy a thousand years ago, it wouldn’t be a surprise if he knew that such medallions existed. That’s why during the Arrancar arc, he had to make a calculated risk on which captains can afford to reveal their Bankai, and which once are too deadly to be stolen. This is where the importance of Gin’s intel comes in.
I guess that’s why it’s excusable that in the Arrancar Arc, we didn’t get to see some of the more interesting Bankai out there. One could only imagine the devastation if such abilities as that of Unohana’s, Urahara’s, or Shunsui’s were to be taken away by the enemy. The only flaw in Yamamoto’s plan is that he was not expecting the existence of Loyd and Royd, who can mimic the appearance and memories of Juha Bach (the only person that can handle the immense power of Yamamoto’s zanpakuto, Ryujin Jakka).
All of these speculations add another layer to the already complex character named Gin. Though it is still unclear what motivated him to dedicate most of his life spying on Aizen, some fans think that he finds Aizen's existence to be a big threat to Soul Society. While others like me believe that he wanted to get back at him for what his men did to Rangiku back when they were younger.
As a final remark, I leave you with one of my most favorite Bleach quote:
“If you were to turn into a snake tomorrow and begin devouring humans, and from the same mouth you started devouring humans, you cried out to me ‘I love you,’ would I still be able to say ‘I love you’ the same way I do today?”
Welp, that’s it everyone. Thanks for reading! ^_^