Soken Ishida is the grandfather of Uryu Ishida and father of Ryuken Ishida. Although he had a close bond with the former, it’s the complete opposite to the latter. Because he is constantly not around during Ryuken’s childhood, the two had grown distant. But it was never specified as to what kept him busy during those years, aside from being a doctor.
At an early age, Soken has taught Uryu the ways of the Quincy – techniques, innate abilities and equipments. And according to Uryu, he was trying to make arrangements with the Shinigami so they can work together to defeat Hollows. But the very nature of the Quincy power prevented that from happening. Could Soken have known about Juha Bach and his second coming? More importantly, could he have known Uryu’s potential to become the second “A”? Throughout the series, it had been implied that Soken knew a lot of things that are relevant to the Stern Ritters – the Sun Gate, Quincy Armor, and the Silver Blood Clot. But how could he have known about Juha Bach’s only weakness?
My best guess is that he was a part of the Stern Ritters. It makes sense – how he was always AWOL on Ryuken’s childhood, why he was eager to train Uryu. And, maybe, just maybe, the first time the Silver Blood Clot was used was a thousand years ago. It would explain how someone as powerful as Juha Bach can be defeated by the first Gotei 13. And maybe, Soken was there to witness it. Besides, we have seen how Bazz B and Hashwalth were able to not grow old after those years.
Welp, thanks for reading anyway ;)