A former pirate captured by Jinzaburou Kuchii before the Mongol invasion, Onitakemaru harbors initial resentment towards his captor. Exiled to Tsushima Island alongside criminals and disgraced individuals, he joins the effort to bolster the island's defenses.

Physically imposing, Onitakemaru wields extraordinary strength exceeding normal human limits. His sharpened teeth contribute to an almost demonic or oni-like appearance. He fights primarily with a kanabō, a large metal war club, reinforcing his association with traditional oni imagery. His combat prowess includes feats like effortlessly snatching an arrow from the air with his teeth.

Initially distrustful and antagonistic due to his capture, Onitakemaru gradually develops mutual respect with Jinzaburou through shared combat against the Mongols. This dynamic culminates in episode 11 when Onitakemaru chooses to return to the island instead of escaping. They fight back-to-back against overwhelming Mongol troops in a critical moment of cooperation.

His fate aligns with the series' high mortality rate. Onitakemaru is listed among those who perish during Tsushima's defense. The ending theme sequence symbolically foreshadows his death as one of the last to fall.