He trained Uryū and Ryūken in Quincy abilities throughout his childhood, becoming his mentor in the process. He was a kind old man whom Uryū admired. He preached an unusual variant of Quincy philosophy advocating cooperation with the Shinigami. Sōken tried to make amends with the Shinigami by advocating a new system of response to hollow attacks: Quincy would be the first-response squad, protecting humans from hollows until the Shinigami arrived to dispatch the hollow, thus maintaining the spiritual balance between the worlds that was being destroyed by the Quincy.

However, his proposal was never accepted, despite several attempts. He was eventually killed in a hollow attack, as the Shinigami arrived to late to save his life. During the rescue of Rukia Kuchiki, Mayuri Kurotsuchi reveals to Uryū that this was his doing; he delayed the Shinigami meant to protect Sōken so that he could study Sōken's soul for his research.

According to a reflection of a photograph seen in Uryu's glasses, all that was left of him in the end was his head. His death is the primary reason Uryū hates the Shinigami, although after learning of the conspiracy Uryū's hatred has become less for Shinigami in general than for Mayuri in particular, despite the fact that the Captain saved his life during the assault on Hueco Mundo. Soken was also seen during the anime only bount arc as a vision to Uryu, where he told Uryu to rest.

(Source: Bleach Wiki)