Hanayo Koizumi is a first-year student at Otonokizaka Academy and a member of the school idol group μ's, later joining its subunit Printemps. Her image color is green or teal. Living with her parents and grandparents, Hanayo's mother was once an idol herself, sparking Hanayo's fascination with idols from childhood. She shares an exceptionally close bond with her best friend since childhood, Rin Hoshizora, who consistently supports and encourages her. Despite harboring a lifelong dream of becoming an idol, Hanayo initially lacked the confidence to pursue it openly and struggled to express her desires, needing significant urging from both Rin and Maki Nishikino to finally voice her wish to join μ's.

Characterized by extreme shyness, quietness, and low self-confidence, Hanayo rarely voices opinions, often defers to others, and startles easily. This timidity affects everyday situations like speaking up in class. However, joining μ's sparks notable growth in her confidence, demonstrated early on by her decision to switch from glasses to contact lenses. Her personality transforms dramatically when discussing school idols or food—especially her intense passion for white rice and rice balls with fillings like kombu, plum, and minced salmon. This enthusiasm sometimes leads to worries about her weight, as seen when she attempted a diet.

Her hobbies include drawing, origami, and dedicated research on school idols, stemming from her time as a devoted fan before joining μ's. She serves as caretaker for the school's alpacas and becomes president of the Idol Research Club after the third-year members graduate. Often described as gentle, kind-hearted, and considerate of her friends' feelings, Hanayo exhibits resilience, persisting through initial fears to perform and gradually becoming more assertive within the group. In-universe vital statistics rate her highly in studies (★★★★) and rice-related enthusiasm (★★★★★), but lower in reflexes (★★) and aggressiveness (★), with maximum conformity (★★★★★).

In *The School Idol Movie*, Hanayo participates in μ's's final activities, including their Love Live competition performance in New York. Her dedication remains evident as the group faces disbandment following the third-years' graduation. The film highlights her ongoing passion for idols and camaraderie with Rin. Her journey culminates in μ's achieving legendary status, cementing her growth from a timid fan into a confident performer.