Tarōja Onimaru is a half-demon, half-human Jukondō master specializing in controlling evil spirits and utilizing forbidden curses. His guardian deity, Mahakala (Daiankokuten), grants him unique spiritual abilities. His dual heritage provides superhuman strength and regenerative capabilities.

Initially a mercenary driven by financial gain, Onimaru operates independently, accepting payment from demon hunters or dark organizations without inherent loyalty to either side. His first encounter with Kujaku establishes them as adversaries, reflecting his opportunistic nature. Their rivalry evolves into a complex alliance through collaborative efforts against mutual threats, leading to Onimaru becoming Kujaku's recurring reinforcement in battles against supernatural forces.

Onimaru transitions from a self-interested mercenary to a reliable, albeit unconventional, ally within Kujaku's circle, despite his initial portrayal as a renegade warrior willing to leverage forbidden arts for personal gain. His involvement proves pivotal across story arcs involving entities like the Teachers of Eight Leaves and modern antagonists such as neo-Nazi groups seeking occult power. His expertise in spirit manipulation and combat is instrumental in thwarting large-scale threats, including preventing dark deity resurrections and the misuse of artifacts like the dragon orb.

While his background lacks extensive elaboration, his actions consistently highlight his adaptability and gradual integration into the core group opposing cosmic and demonic dangers.