Captain of the 7th Discipline Squad, Mayumi is Kōtarō's childhood friend and chief romantic interest. She's the only one who seems to be able to keep Koutarou in check. While she is nowhere near Kōtarō's level of strength, she is not a damsel in distress. She has got a good grip on the basics of both karate and judo. Mayumi also happens to be a member of the school's Decency League, much to Kōtarō's annoyance.