Nanao Inusaka is a Bushi, a warrior who fights alongside the Oni demons rather than against them. She belongs to an unknown organization whose goal is to capture the Obsidian Goddess. Physically, she is a young woman with a small build, middle-length burgundy hair, and dark eyes. Her personality is generally indifferent, with a gloomy expression. She takes her missions seriously and can be merciless on the battlefield, but she prefers to finish her tasks as quickly as possible so she can return home and lounge around. Despite a composed exterior, she has an absent-minded side.

Her primary motivation is to follow the orders of her superior, Inukai Shiro, whom she calls master, and to serve the goals of her organization. She acts as a calm and accurate critic toward Shiro while also handling his daily needs. She initially develops an interest in Musashi because he was kind to her when they first met, which later leads to feelings of guilt after she deceives him in combat.

In the story, Nanao is a member of the Obsidian Eight, a group of powerful Bushi aligned with the demons. She first appears as an antagonist, sent to capture or eliminate Musashi and his allies. During a confrontation, she uses her kitetsu blade, the Seven-Belled Armadillo Sword, a staff that can split into a seven-sectioned sword and be wielded like a whip. She also possesses a mysterious ability referred to as the Green Sword Soul. In battle, she initially has the upper hand, but after the Obsidian Goddess intervenes and forces a retreat, Nanao is left grappling with lingering guilt over having tricked Musashi. This suggests potential for development as she reconsiders her loyalties and actions.