Onimaru stands as a dark counterpoint to the protagonist Kujaku, wielding shadowy mystical arts. His nature remains ambiguous, possibly demonic or half-human, with no defined origin or history. Primarily a pragmatic mercenary, he offers his services for payment, aligning with allies or antagonists as profit dictates.

His abilities focus on supernatural combat and the occult: employing dark magic, curses, and spiritual energy attacks. He demonstrates proficiency in summoning creatures, utilizing necromancy to raise ancient warriors, and casting hypnosis to manipulate others. He also possesses significant physical prowess, skillfully wielding bladed weapons in kinetic confrontations and often arriving dramatically on a motorcycle during critical action.

Character development is minimal; he steadfastly maintains an opportunistic and morally flexible stance, showing no significant evolution in motives or personality. His role is primarily functional, driving conflict or aiding resolutions through shifting allegiances rather than personal growth. In later storylines, like the 1994 OAVs, he participates in battles against cosmic threats such as resurrected dark gods and neo-Nazi occultists, motivated by transactional interests, not ideology.

Interpersonally, he shares a Yin-Yang dynamic with Kujaku, embodying shadow to the protagonist's light. While occasionally cooperating with Kujaku's group against greater dangers, their interactions remain tense and strictly alliance-driven. Relationships beyond these tactical alliances, such as with the sidekick Ashura, remain unexplored.