Mutsuki, also known as "Mucchii", is an exceptional athlete whose reputation often precedes her. In effect, sports clubs at school have invited her to participate in several club activities as an honorary member. Not wanting to turn them down, Mutsuki ends up participating in a multitude of other sports-club related activities in addition to the work she puts forth for the student council, despite initially not being an official member. This is due to her liking to help out where she is needed. She has a tomboyish personality, often hitting Mei on the back very hard and sporting a short haircut for a girl. Mutsuki's mannerisms are not feminine either and her voice is deeper than the typical female voice for her age.

Mutsuki has known Mei Etoh as far back as elementary school and often tries to get her to participate with the other main characters as a group rather than distance herself more from them. Mutsuki was the first person Mika became friends with upon entering high school. The two share an odd friendship as they have little in common. Mutsuki is eventually inducted into the student council as the assistant.