Renzo's father was a member of the Itto-Ryu under Araya Kawakami. He starts off befriending Rin, but here's the dark twist—his own father is revealed to be the one who raped and murdered Rin's mother. Knowing what would happen if Renzo ever found out the truth and sought revenge, Rin and Manji stage a scene making it look like Manji killed Araya in cold blood, then have Renzo seemingly kill Manji in retaliation. It's brutal, but it spares Renzo from becoming consumed by the need for vengeance the way Rin was.
After being orphaned by Manji, Renzo ends up working alongside Shira.
Content compiled by AnimeList.moe from publicly available sources.
