Kumiko is a young teenage girl with an average height and build. She has light auburn hair which is shoulder length, amber eyes, and fair skin.
Within the story of Sound! Euphonium, Kumiko is an ordinary girl to which interesting things happen. She is surrounded by characters that drive or instigate dramatic situations. 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. As a willing and nonjudgmental listener, she easily befriends everyone she meets and accepts both their flaws and strengths. 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.
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. 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. 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.
Kumiko speaks standard Japanese, which is rare in Kansai. Her school bag is plain, and it seems that she is not interested in anything "over-the-top". She is an indoors type of person and she does not get along with sports-oriented people, except for Hazuki. In addition, Kumiko sometimes compares herself to others too frequently. This is why she often puts up a "good-girl" face, but in reality, she is actually really distant.
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.
Kumiko's older sister, Mamiko, is one of the reasons behind Kumiko's interest in playing low brass instruments. 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. However, the band was unable to qualify for the national-level tournament. The results of the competition created a rift between her and Reina. 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.
On her first day in Kitauji High School, Kumiko learns of the school's own concert band and comments on its dismal quality. Meanwhile, she meets new friends in her class, namely Hazuki Katou and Sapphire Kawashima, the latter revealing her music background and her interest in joining the school band. Many things prevent Kumiko from joining, including her experience in the middle school prefectural tournament, and the fact that Reina herself, who also enrolled in Kitauji High School, has just revealed her interest in joining the school band. Eventually, after much thought and seeing the eagerness of both Hazuki and Sapphire in joining the band, Kumiko decides to join as well, but not until she could resolve her past with Reina.
- The name Kumiko means "long time" (久) (ku), "beautiful" (美) (mi) and "child" (子) (ko).
- According to the official Sound! Euphonium novel website:
- Kumiko likes egg dishes, pastry, rice omelet and shortcake, and dislikes insects, especially caddisflies
- Her favorite colors are white and yellow.
- Her hobbies are listening to music and lazing around at home.
- Her special skill is being able to drink a can of corn soup without leaving a grain.
- It was shown at least twice that Kumiko plays bass clef for her sheet music. It was also shown in the key of B♭ Major, indicating that Kumiko plays a non-transposing instrument.
- Kumiko likes novels which contain extreme matter that contrasts to her character.
- Kumiko lives on the south bank of Uji River, near Byoudouin Temple and Agata Shrine.
- Kumiko is very insecure about her below-average chest size. She tends to compare herself frequently during the series the books and the novels.
- In her second year, Kumiko is appointed, along with Tomoe Kabe, to be an instructor for the new band members. She also founds a counselling center.
- Kumiko's euphonium is based on a Yamaha YEP-621 model.