Kumiko enrolled in Kitauji High School to wear the sailor uniforms she liked and to get a fresh start. Though she was doubtful, she decided to join the concert band club and continue playing the euphonium. She has a habit of immediately saying what she’s thinking and changes her mind easily. She had a falling-out with Reina in middle school.
After Reina told her that she wants to be special by playing the trumpet, Kumiko later decided to become special too.
Kumiko initially joined band because she admired her sister Mamiko. She continues to play the euphonium, despite concerns about studying, because she does not want to follow in the steps of her sister.