Mutsuko is one of three elementary school girls visiting St. Claire Elementary who discover living exhibits in the science room. She possesses persistent curiosity and sharp observational skills, frequently focused on capturing specimens, especially a fly she pursues relentlessly throughout the night. Her quiet, analytical demeanor makes her the group's most focused member, though her intense single-mindedness can become unsettling.

Her appearance features long bangs perpetually obscuring her eyes, creating an enigmatic presence accentuated by an oversized dress. This visual design amplifies tension when her hidden gaze suggests thoughts prioritizing her own interests over others' discomfort.

Mutsuko demonstrates a tendency for hands-on experimentation with little regard for consequences. This is shown when she captures the rabbit character Sonny, binding him and carrying him tribal-style on a flyswatter. Similarly, she disregards the fly's aversion to capture, reflecting her detached observational approach.

During the Music Room challenge, where the girls must compose a melody to impress guardian portraits of composers, Mutsuko's accidental action proves crucial. While trying to recapture her escaped fly, she forcefully strikes a single piano key, producing a resonating B-flat. This unintended minimalism unexpectedly satisfies the composers and secures one of the required medals.