Yukiho Kosaka serves as Honoka Kosaka's younger sister and a supporting character in the anime series. Initially drawn to UTX High School's popularity and modern image, she considers Otonokizaka High outdated and destined for closure, indirectly motivating Honoka to establish the μ's idol group to revitalize it.

Pragmatic and observant, Yukiho contrasts Honoka's impulsiveness. She exhibits deep concern for her sister's well-being, notably rushing without her umbrella to aid Honoka when she collapses after a rain-soaked performance. Though frequently embarrassed by Honoka's intrusions into private moments, their bond remains strong. Yukiho scolds Honoka for neglecting health but also offers reflective insights, such as questioning μ's future after the third-years graduate.

Her perspective evolves from dismissiveness to active support. She acknowledges μ's power to inspire others, confessing they make her genuinely root for them beyond familial duty. This culminates in her enrollment at Otonokizaka alongside friend Alisa Ayase, symbolizing the school's renewed appeal. Later, she convinces Alisa to form a new idol unit instead of joining μ's, recognizing μ's legacy belongs to its original members—a conversation helping Honoka accept the group's eventual disbandment.

Yukiho shares a close friendship with Alisa, frequently attending events together or discussing idol activities. They collaborate to create their own idol unit upon entering high school. Occasionally, Yukiho engages in self-conscious behaviors like struggling with outfits or worrying about body image, reflecting her awareness of contemporary trends.

Residing with Honoka and their parents at the family-run traditional Japanese sweets shop, she values practicality and academic diligence, consistently dedicating time to study.