Karyuu is a highly skilled Edo-period ninja operating when demand for her kind has waned. Forced to accept work as a hired killer, she takes an assignment to assassinate a merchant. At the target's location, she discovers the merchant's servant, Uzume, is also a ninja of comparable skill. This encounter ignites a violent, ongoing rivalry as they engage in repeated confrontations to establish dominance.

Her background reflects the era's peace, transitioning her from traditional ninja roles to mercenary work. Known for combat proficiency, particularly with swords, Karyuu embodies traits of a ruthless assassin. The conflict with Uzume forms the central focus of her story, driving events across the media's episodes.