Oniwakamaru is a large man with a misshapen face that frightenes most people. Oniwakamaru is haunted by memories of being chased out of a village by a torch wielding mob, and is what motivates his defensive violence. He wields a large double bladed weapon and is strong enough to knock a defending fighter off their feet.

In episode 2, Ryujiro frees Oniwakamaru from jail and manipulates him into becoming his bodyguard while Ryujiro exacts his vengance upon Mugen, Jin, and Fuu. Ryujiro "controls" Oniwakamaru's action by bringing up his past, for instance saying that the prefectural police who are after Ryujiro are actually going to hurt Oniwakamaru like the villagers did, causing Oniwakamaru to kill both officers in a blind rage.