Sōken trained both Uryū and Ryūken in Quincy abilities and became Uryū's most important mentor. He was kind and genuinely cared about his grandson, who deeply respected him. Unlike most Quincy, Sōken believed cooperation with the Shinigami was possible—even necessary. He pushed for a new approach where Quincy would act as the first line of defense against hollows, protecting humans until the Shinigami arrived to finish them off. This system, he argued, would restore the spiritual balance that Quincy extermination was destroying. Despite his efforts and multiple proposals, the other Quincy rejected his vision.
A hollow attack killed Sōken before the Shinigami could arrive to save him. Years later, during the arc where Rukia's rescue is being planned, Mayuri Kurotsuchi reveals the ugly truth to Uryū: he deliberately delayed the Shinigami response team so he could study Sōken's soul for his experiments. Very little of Sōken remained after the attack—Uryū later sees only his head reflected in his own glasses.
Sōken's death became the foundation of Uryū's hatred toward the Shinigami, though that anger eventually focused less on Shinigami as a whole and more specifically on Mayuri himself. It's worth noting that despite this grudge, Mayuri ended up saving Uryū's life during the Hueco Mundo arc. In the anime-only Bount arc, Sōken appeared to Uryū as a vision, encouraging him to rest.
Content compiled by AnimeList.moe from publicly available sources.
