Hazuki Katō is a first-year student at Kitauji High School who quickly becomes one of the central figures in her friend group. She is easily recognized by her short brown hair, which she often styles with a red or pink hair clip, and her gold-hazel eyes. Standing at 155 cm tall, she has a healthy, athletic build and a slight tan, a lasting remnant of her time on the tennis team during middle school.

Her most defining trait is her boundless energy and infectious optimism. Hazuki is a natural mood-maker, always cheerful and quick to befriend others, as seen when she starts calling the protagonist Kumiko by her first name moments after they meet. She is straightforward and honest, often speaking her mind without a filter, a quality that can be both refreshing and startling to her more reserved peers. Though she carries herself with confidence, she has a charmingly vulnerable side, such as being easily frightened by ghost stories. She prefers comfortable, sporty clothing over frilly or feminine outfits, feeling that they do not suit her active personality.

Hazuki enters high school with no musical experience, having been drawn to the concert band simply because it looked like fun. Her initial dream is to play the trumpet, but due to a comical mix-up involving instrument mouthpieces, she ends up as a member of the bass section, tasked with playing the large and heavy tuba. Rather than be discouraged, she embraces this new challenge with characteristic enthusiasm, even giving her instrument a unique nickname, the Chupacabra. Her motivation stems from a desire to enjoy the present moment and a refusal to dwell on worries or regrets, a philosophy that helps her push through difficult times.

Within the story, Hazuki serves as a close friend and classmate to the protagonist, Kumiko Oumae, and is a core member of their trio with Sapphire Kawashima. This friendship is the heart of her social world, and she is often a source of support and encouragement for them. She also forms a significant, albeit one-sided, romantic attachment to Shuuichi Tsukamoto, Kumiko's childhood friend. Her feelings culminate in a direct confession at a school festival, where she is gently rejected, as Shuuichi has feelings for Kumiko. This event is a major turning point, showcasing her emotional strength; despite the pain, she resolves to move forward and supports her friends' eventual relationship.

Her primary relationship in the band is with her instrument, but she is also mentored by the charismatic and eccentric Asuka Tanaka, the bass section leader, who inspires Hazuki's passion for the tuba. While initially intimidated by the talented and serious trumpeter Reina Kousaka, Hazuki eventually comes to respect her, learning from her strict but helpful advice.

Over the course of her first year, Hazuki undergoes notable development. Initially frustrated by her lack of skill, she learns the value of perseverance. She fails to qualify for the main competition ensemble as a first-year, becoming a supporting member instead. However, she does not let this setback defeat her. She dedicates herself to practice, and by her second and third years, her abilities have improved so significantly that she is considered a reliable and competent player, even earning a spot in the national competition ensemble and being chosen to lead the bass section and guide new first-year students. Her journey is one of transformation from a complete novice who knew nothing of music to an experienced senior performer and leader, all driven by her unwavering, cheerful determination.