Mutsuki Uehara, nicknamed "Mucchii" or "Mucchi," is an exceptionally skilled athlete. Her reputation prompts multiple sports clubs at Seioh Private High School to invite her as an honorary member. Driven by her willingness to help, she accepts, participating in numerous club activities alongside student council work. Though not initially an official member, she contributes to the council due to her inclination to assist others.

Her personality is notably tomboyish, expressed through energetic physical gestures like hitting Mei Etoh hard on the back. She sports a short haircut uncommon for girls her age, and her mannerisms lack traditional femininity. Her voice is deeper than typical for her female peers.

Mutsuki has known Mei Etoh since elementary school and consistently encourages her to join their peer group instead of isolating herself. Upon Mika Inamori's entry into high school, Mutsuki formed a friendship with her, establishing an unconventional bond despite their differing personalities and interests. This friendship marks Mutsuki as Mika's first friend at the school.

Over time, Mutsuki's student council involvement formalizes, and she is officially inducted as the assistant to the student council president. Her role within the council and her relationships with fellow students, particularly Mei and Mika, form a consistent part of her background and interactions.