Mutsumi Mitsuba is a first-year student at Osai Academy who later advances to the second year. She is the founder and leader of the school's judo club. In her first year she shares a class with Takatoshi Tsuda and Kenji Yanagimoto, and in the second year her classmates include Suzu Hagimura, Nene Todoroki, and her best friend Chiri Nakazato. Mutsumi has long black hair that reaches the middle of her back, which she always wears in a ponytail, and she is in excellent physical condition. Her personality is energetic, kind, and cheerful, making her a natural moodmaker who easily befriends both boys and girls. She is also notably innocent and pure-hearted, often misinterpreting the risqué comments of student council members Aria Shichijou and Shino Amakusa as something wholesome. Mutsumi is hardworking and deeply dedicated to her judo club, where she demonstrates strong fighting skills and leadership. However, she struggles academically, finding it hard to sit still and frequently dozing off in class. Her motivations are simple: she wants to lead a successful judo club, and later in the series she develops a crush on Takatoshi Tsuda, expressing that her dream is simply to become a bride. Her key relationships include her close friendship with Chiri Nakazato, who often tries to correct her, and her reliance on Takatoshi or Suzu to explain the student council's conversations. Over time, Mutsumi's innocent nature remains largely unchanged, but she shows a subtle romantic interest in Takatoshi, even playfully combining their names on a blackboard. Her notable ability is her exceptional physical strength and skill in judo, which makes her a formidable fighter, though she remains simple-minded and naive about many everyday matters.