Nobume Imai, formerly known as Mukuro, is a former member of the assassination unit of the Mimawarigumi and its former vice captain, later becoming the director of the police department. Her origins are rooted in a harsh past: she was an orphan taken in by the Tenshouin Naraku and trained from a young age to become a professional assassin. This conditioning left her with an emotionless demeanor and a flat, monotone manner of speaking, yet beneath that surface lies a pronounced sadistic streak. In combat she becomes a relentless force, ignoring everything until every target is eliminated. Her only visible indulgence is a near‑obsessive love for doughnuts, particularly Pon de Ring, for which she will resort to violence without hesitation.

Nobume initially serves as a secondary antagonist during the Thorny Arc, acting under the orders of Isaburo Sasaki, the leader of the Mimawarigumi. She later develops a more complex role, including guarding Princess Soyo and eventually succeeding Sasaki as head of the police department. Over the course of the story she begins to show a greater range of emotion and loyalty, notably forming an informal bond with Kagura and Princess Soyo—a trio sometimes called the Sisterhood Sadist Three. She also shares a tense but significant rapport with Gintoki Sakata, whose influence subtly softens her rigid worldview.

Her abilities as a swordsman are exceptional; her skill is described as comparable to a grim reaper, placing her among the most dangerous fighters in the series. She wields a katana with lethal precision and is capable of single‑handedly dispatching multiple opponents. Throughout the narrative, Nobume’s personal growth is tied to her gradual emancipation from the Naraku’s conditioning and her emerging sense of identity beyond that of a mere tool. By the end of her arc, she has become a stabilizing force within Edo’s police force, carrying the responsibilities left by Sasaki while retaining the deadly competence that defined her past.