Childlike, ogreish killing machine sprung from prison and recruited by Ryūjirō to bring Mugen to his knees. Uses a weapon similar to Tessai's from Ninja Scroll (and resembles him, interestingly). He has been feared and ostracized all his life due to his size and deformities, until he is shown kindness by Fuu. He easily defeats Mugen with his immense strength (At one point he throws Mugen through a wooden fence). He strangles Ryūjirō to death after Ryūjirō threatens Fuu's life. Following that, he and a severely poisoned Mugen engage in combat again. Fuu screams for them to stop. Oniwaka does stop, but Mugen doesn't. Oniwaka is then impaled by Mugen's sword and dies with Fuu at his side.

It is never specified how Oniwaka obtained his weapon and who taught him to wield it with such skill, or whether he taught himself, given his status as a pariah. Additionally, "Oniwaka" was the moniker of Saito Musashibo Benkei, an actual person but better remembered as a legendary figure in Japanese folklore of great strength and stature.

Watanabe has admitted that Oniwaka, while similar to the cinematic Frankenstein's monster, is actually a reference to zombies. The entire episode in fact was a reference to the samurai zombie genre.