Shin'etsu Kisaragi

was a Shinigami from the First Division and the father of Shūsuke Amagai.

Synopsis
Little is known about Shin'etsu, other then he was killed by Shigekuni Yamamoto-Genryūsai. Amagai knows about that and he figurates that the reason of Shin'etsu's assasinate, was because he knew about the existence of the Bakkōtō, and Yamamoto's possible involvement with the Kasumiōji clan to secretly allow them to create Bakkōtō, as his father's last words were "Beware of the Bakkōtō". As a result, Shūsuke Amagai fraguates a plan against Soul Society to avenge his father. In the final moments of the last battle between Ichigo and Amagai, Amagai is set to kill Yamamoto, but Yoruichi Shihōin tells Amagai that his father was another victim of the bakkōtō, revealing him that Shin'etsu was one of the loyalest and greatest pupils of Yamamoto, and that he wants to took the mission to undisclose the evil plans of Gyōkaku Kumoi, who used his father as an experimental body and sents him to kill Yamamoto, he were possesed by the power of the Bakkōtō, so Yamamoto took the hard decission to kill his subordinate, freeing his soul from the evil katana.