On her first day in Kitauji High School, Kumiko learns of the school's own concert band and comments on its dismal quality. Since then, the confrontation during the end of the prefectural tournament continues to haunt her as she tries to create a fresh start for herself in Kitauji High School. The results of the competition created a rift between her and Reina Kousaka. However, the band was unable to qualify for the national-level tournament. Kumiko has been a well-experienced euphonium player since middle school, and was a member of the Daikichiyama Middle School Band, which was able to secure a gold award in the prefectural (first round) tournaments. Mamiko Oumae is one of the reasons behind Kumiko's interest in playing low brass instruments. Over the course of the series, Kumiko grows more passionate about music and life in general, training hard to play the euphonium flawlessly as she finds the motivation and strength to become a better musician and better person. This is why she often puts up a "good-girl" face, but in reality, she is actually really distant. In addition, Kumiko sometimes compares herself to others too frequently. She also has a habit of dwelling on past situations to the point of obsessing over them in her head, such as her awkward encounter with Reina prior to entering high school. This lack of a filter is part her impulsive streak, a trait which also led her to enroll at Kitauji for a reason as trivial as its' uniform design, and also to accidentally ask Reina Kousaka to attend the Agata Festival with her. Kumiko is mostly friendly and polite, but she can be awkward and blunt sometimes due to her quirky habit of accidentally blurting out the first thing on her mind without restraint or tact. However, Kumiko dislikes her own self because she knows she is a poor communicator who has difficulty making her wishes known to others, and so often listens instead of speaking. As a willing and nonjudgmental listener, she easily befriends everyone she meets and accepts both their flaws and strengths. Although she occasionally has a hand in the events around her, Kumiko mostly plays the role of a bystander, witnessing enough situations and exchanges that she receives both sides of a story. She is surrounded by characters that drive or instigate dramatic situations. Within the story of Sound! Euphonium, Kumiko is an ordinary girl to which interesting things happen.
Kumiko is a young fair-skinned teenage girl with shoulder-length light auburn hair that is curled at the bottom, amber eyes and an average height/build.