Hazuki Katō is a cheerful and energetic second-year student at Kitauji High School, playing tuba in the concert band. She has short brown hair often adorned with a red-pink hair clip, gold-hazel eyes, and a slightly tanned complexion from her past athletic pursuits. Standing 155 cm tall, she wears the standard female school uniform and favors sportswear like orange and yellow hoodies and sneakers.

A former tennis club member at Higashi Middle School with no musical background, Hazuki joined the high school band hoping to play trumpet but was assigned tuba due to her strong lungs. She affectionately named her school-issued Yamaha YBB-641 tuba "Chupacabra." Her outgoing personality makes her a social mood-maker within the low brass section, quickly befriending classmates like Kumiko Oumae and Sapphire Kawashima, whom she addresses by first name.

Despite initial struggles as a beginner, she practiced fundamentals like long tones diligently. As a first-year, she was placed in the backup "Team Monaka" for competitions, embracing this supportive role and learning the value of teamwork. By her second year, she mentored new tuba players Satsuki Suzuki and Mirei Suzuki, becoming vice-section leader under Sapphire. She developed a crush on Shūichi Tsukamoto, Kumiko's childhood friend, but accepted his gentle rejection and supported his feelings for Kumiko.

In her third year, Hazuki assisted in training first-year students while grappling with future career decisions. After self-reflection and guidance from teacher Michie Matsumoto, she pursued nursery teaching, leveraging her aptitude for caring for others. She retired from the band after her final competition to focus on college entrance exams, later rejoining briefly for a senior class trip performance in Okinawa, where she played a tuba solo. Hazuki also appears in a minor supporting role as a member of the concert band during the events of *Liz and the Blue Bird*.