Mirei Suzuki is a first-year student at Kitauji High School, one year below Kumiko Oumae, and a tuba player in the school's concert band. She first appears in the series during Kumiko's second year. Mirei is tall with fair skin, short black hair parted on the left, and blue eyes. She is known by the nickname Micchan, a holdover from her elementary school friendship with Satsuki Suzuki, though she dislikes being called that and becomes visibly upset when addressed that way.

Mirei comes from a strong music background, having been a member of a top middle school band for all three years. Her skill on the tuba is exceptional, surpassing even that of some upperclassmen when she joins the club. She approaches practice with a calculated, efficient style and is known for her straightforward honesty, which earns her both admiration and wariness from her peers. She takes everything she does very seriously and holds herself to high standards, often displaying perfectionist tendencies. Her personality is sharp and direct; she tends to be stoic and distant initially, and she has a strong aversion to unwanted physical contact, as seen when she instinctively slaps away Hazuki Katou's hand during the Sunrise Festival.

Mirei's core motivation is a desire to be judged on equal footing with everyone else, as expressed in her wish to stand on the same starting line as her peers. She is principled and has a strong sense of right and wrong. Despite her outward reserve, her love for music is evident in her dedication to the band. Her early social awkwardness and fear of opening up to others serve as a barrier, but a key turning point arrives when she becomes involved in a tense argument with her friends, forcing her to confront her emotional walls and leading to personal growth. Over time, she learns to be more open and collaborative.

Within the band, Mirei is a vital member of the tuba section and later takes on the role of drum major, leading rehearsals and playing a significant part in the ensemble's performances. She also helps train younger members, demonstrating her reliability and leadership potential. Her primary relationships include her childhood friend Satsuki Suzuki, with whom she shares a complicated history, and the other first-year band members like Hazuki Katou and the rest of the low brass section. Mirei's development in the story is centered on overcoming her antisocial tendencies, embracing teamwork, and using her considerable musical talent to contribute to the band's collective goal of reaching the national competition.