Kumiko Ōmae plays euphonium in the Kitauji High School concert band. She joined reluctantly, burdened by unresolved tensions from middle school. There, a fallout with Reina Kōsaka followed their group receiving a "dud gold" award and failing to reach nationals. Characterized by initial indecisiveness, Kumiko frequently questions her own motivations, leading to contemplative reflection. She struggles to express her feelings socially and can be blunt or awkward in confrontations.

Her family includes parents and her older sister, Mamiko, who first inspired Kumiko to play euphonium. Their relationship strains when Mamiko considers dropping out of college, leading to arguments. They later reconcile after Kumiko learns Mamiko regretted abandoning music, helping Kumiko understand her own dedication.

Within the band, Kumiko evolves from an uncertain first-year into a confident leader. In her second year, she involves herself in resolving conflicts: advocating for Nozomi Kasaki's reinstatement, mediating Mizore Yoroizuka's anxieties, and observing Asuka Tanaka's facade. She discovers Asuka's estranged relationship with her father, a famous euphoniumist judging nationals. Kumiko pleads with Asuka to stay, warning quitting brings regret. This helps Asuka negotiate continued participation using academic achievements as leverage with her mother.

Kumiko's friendship with Reina Kōsaka profoundly impacts her. Reina's trumpet dedication inspires Kumiko's musical pursuit, and shared moments—like climbing Mount Daikichi or visiting Noboru Taki's wife's grave—deepen their bond. Kumiko also helps Reina navigate feelings for Taki by eventually disclosing information about his late wife.

As third-year band president, Kumiko manages new challenges, including potential first-year boycotts. She addresses issues personally, reassuring Sari after illness and advising peers like Motomu on personal struggles. She contemplates her future career path, discussing options and considering music school after Reina's suggestion, though uncertain about making a living as a musician.

Her leadership supports underclassmen and maintains band cohesion during competition preparations. Her efforts contribute to the band winning a bronze medal at nationals. After third-years graduate, Kumiko expresses admiration for Asuka, who gifts her a musical piece titled "Hibike! Euphonium," composed by her father.

Kumiko's hobbies include listening to music and spending time at home. She enjoys foods like egg dishes, pastries, rice omelets, and shortcake.