A huge warrior who went around relentlessly killing anyone. Because of this, people started calling him a demon. One night, as he was slaughtering an entire village, he was unable to kill one child. From this night on, he only attacked samurai's and warriors who were worthy. This child was Zankuro's daughter, Shiori. Eventually, he was challenged by a boy who also had a demon inside of him. Zankuro was killed by Shizumaru and was buried and sealed by Gaira. But he was quickly revived by Amakusa who wanted to use his powers. He was eventually killed again by Sougetsu. It seems he somehow came back to life again since he appears in Genjuro's ending in ST1.

Miscellaneous Facts and Notes

Based off of Isurugi Raijyuta from "Rurouni Kenshin".
(the developers admitted to this in an interview)

He is not Shizumaru's father.

Misuzu - his wife(dead)
Kakunoshin - his son(dead)
Shiori - his daughter