He trained Uryū and Ryūken in Quincy abilities throughout his childhood, becoming their mentor in the process. He was a kind old man whom Uryū admired greatly. Sōken advocated for an unusual variant of Quincy philosophy that promoted cooperation with the Shinigami. He proposed a new system where Quincy would act as the first-response squad, protecting humans from hollows until the Shinigami arrived to deal with them, thus preserving the spiritual balance between worlds.
However, despite several attempts, his proposal was never accepted. Eventually, he was killed during a hollow attack because the Shinigami arrived too late to save him. During the rescue of Rukia Kuchiki, Mayuri Kurotsuchi revealed to Uryū that he had intentionally delayed the Shinigami meant to protect Sōken so that he could study Sōken's soul for his research.
According to a reflection seen in Uryū's glasses, all that remained of him in the end was his head. His death is the main reason Uryū harbors deep resentment towards the Shinigami. However, after learning about the conspiracy, Uryū's hatred has shifted more specifically towards Mayuri, despite the fact that the Captain saved his life during the assault on Hueco Mundo. In the anime, Sōken also appeared as a vision to Uryū during the Bount arc, advising him to rest.
(Source: Bleach Wiki)
