Onimaru operates as a mercenary, wielding forbidden curses and dark arts magic, and aligns with any side providing payment. He commands significant knowledge of supernatural forces and forbidden magical practices, deploying them against supernatural threats. His approach to spellcasting and combat is strictly pragmatic, prioritizing effectiveness over moral concerns.

He embodies the shadow-oriented, darker aspects of spirituality, functioning as a yang counterpart to protagonists aligned with light-based powers. Onimaru occasionally collaborates with Kujaku, a Tantric Buddhist monk, to confront greater dangers despite their differing methodologies. This partnership exemplifies a recurring dynamic where opposing forces unite against common enemies, such as neo-Nazis attempting to unleash ancient evils or necromancers trying to resurrect destructive entities.

His expertise lies in combating demons and malevolent spirits, frequently employing unconventional or forbidden techniques that contrast with traditional heroic methods. During crises involving artifacts like the Asura statue—which contains a universe-destroying demon—he provides essential dark-magic support for containment. His actions stem solely from transactional interests, not altruism, yet he demonstrates reliability in high-stakes supernatural scenarios.

Onimaru's background and personal history remain unelaborated. His development across official media centers exclusively on his role as a situational ally or antagonist, with no significant evolution or expanded origins documented. His contributions focus on applying dark arts to resolve immediate threats alongside protagonists, without deeper exploration of his motivations or past.