Kumiko Ōmae's musical journey starts under the influence of her older sister Mamiko, an experienced trombone player. This early inspiration leads Kumiko to become a skilled euphonium player with the Daikichiyama Middle School Band. Though they win gold in prefectural tournaments, their failure to advance to nationals creates a rift with future classmate Reina Kōsaka. Initially reluctant to join Kitauji High School's concert band, Kumiko participates after encouragement from friends Hazuki Katō and Sapphire Kawashima. She considers switching instruments but is persuaded to continue euphonium by senior Asuka Tanaka.

During her first year, Kumiko evolves from an indecisive student into a more committed band member. Her reconciliation with Reina Kōsaka becomes pivotal, motivating Kumiko to pursue musical excellence. She earns a competition spot through auditions and helps the band secure gold at the Kansai regional competition, advancing to nationals. She navigates complex family dynamics, particularly a conflict with Mamiko over career choices. They reconcile when Mamiko attends nationals, finding shared understanding.

As a second-year student, Kumiko mentors new first-years Kanade Hisaishi, Mirei Suzuki, Satsuki Suzuki, and Motomu Tsukinaga. She faces challenges including interpersonal conflicts and Mirei's temporary departure due to perceived lack of seriousness. Kumiko assists Kanade during a contentious audition process and supports Tomoe Kabe's transition to band manager following a medical issue. Kumiko and childhood friend Shūichi Tsukamoto mutually end their romantic relationship to prioritize band commitments. The year concludes with the band earning a "dud gold" at Kansai, failing to advance to nationals, witnessed by alumni including Asuka Tanaka.

In her third year as band president, Kumiko balances administrative duties with mentoring new members, including transfer student Mayu Kuroe, a skilled euphonium player from a renowned band program. Her leadership emphasizes empathy and conflict resolution. She mediates issues like a potential first-year boycott and misunderstandings involving siblings Suzume and Tsubame Kamaya, though these efforts leave her feeling inadequate. Reina Kōsaka reassures her, noting their instructor's confidence in Kumiko's abilities. Under Kumiko's presidency, the band selects the demanding piece "Hitotose no Uta" and recommits to winning gold at nationals, reflecting her growth into a resilient leader who channels past experiences into guiding others despite lingering self-doubt.