Hiro Kurusu, a third-year Tokise High School student, enters the Koto Club captivated by their assembly performance. Her sociable, stylish appearance features wavy medium-length light-brown hair swept right, brown eyes, and multiple right-ear earrings. Winter uniforms include a beige vest over a white blouse, red-and-orange striped bow tie, checkered brown skirt, blue socks, and brown shoes; summer swaps the blouse for light blue. She keeps nails trimmed for koto despite preferring longer styles.

Initially joining under deception, Hiro confesses her motive sprang from envy of the club's unity, rooted in junior high trauma where a transferee manipulated friends and a boyfriend into distrusting her. Believing relationships fragile, she aimed to shatter the club by spreading lies: telling members Satowa Hozuki resented accommodating amateurs and disliked Chika Kudo's koto approach. Later, she exposed Satowa’s disownment by the Hozuki group, framing it as proof Satowa saw the club as temporary refuge. These schemes collapsed when members reaffirmed trust in Satowa—Chika instantly rejected Hiro’s claims, while Kurata thanked Satowa for her honesty.

Confronted by Kurata after fleeing the clubroom, Hiro admitted her intent to sow discord. His subdued reply—noting she "must have been really bored"—triggered her tears, exposing unresolved pain. She apologized to all manipulated individuals, accepting physical retaliation without resistance as atonement. This pivotal moment led her to rejoin the club sincerely. Becoming vice-president to ease Kurata’s burdens, her dedication intensified: she practiced rigorously during holidays and volunteered her uncle’s inn for training camps.

Her relationships transformed profoundly. She bonded mutually with Satowa—offering social guidance while receiving koto refinement help. She grew protective of younger members like Kota Mizuhara, blaming herself when he fled training and supporting his growth. With Kurata, initial mockery (using nicknames like "Pansy") shifted to mutual respect and romantic tension. She visited his home to discuss leadership struggles, defended his seriousness as admirable, and flushed when he held her hand confronting her ex-boyfriend. Kurata reciprocated her feelings, acknowledging her change and vowing to listen if she shared her past—though neither formally confessed.

Embracing her role as the club’s emotional anchor, Hiro rallied members during setbacks. She championed Chika and Satowa’s budding relationship with enthusiastic interest and fueled cohesion during national competition preparations, practicing relentlessly to match their collective ambition.