Haruko Saeki, a 20-year-old student at Seiyou Private University, stands as the eldest member of Hoshimi Production’s idol group Sunny Peace. Initially concealing her age from teammates out of self-consciousness, she later confronted this insecurity as part of her growth. Before joining the group, she pursued a solo idol career that faltered, sparking doubts about her abilities. Yet loyalty to those who supported her fueled her resolve to continue.

Warm and approachable, Haruko adopts a nurturing role, offering steady encouragement through small acts of kindness and an unwavering smile. Beneath her composed presence lingers a fear of blending into the background beside younger, more dynamic peers. She finds solace in tending eggplant and tomato gardens, crafting homemade pickles, and solitary camping trips along Hoshimi City’s coast—hobbies mirroring her introspective nature and appreciation for quiet moments.

A childhood encounter with a live idol performance ignited her dream, shaping her belief in teamwork over personal acclaim. Past frustrations from unsuccessful auditions transformed into determination to mentor Sunny Peace’s newer members, prioritizing unity and mutual progress. She shares a bond with Shiraishi Saki and Ibuki Nagisa, forged through shared domestic tasks and sisterly camaraderie, while quietly nurturing an unnamed rivalry she vows to confront only after proving her own growth.

Her journey centers on embracing her seniority, channeling past struggles to guide others while steadily gaining confidence. The lyrics of "Shine Purity"—"Look at the stars and let them shine, I'm not afraid of the dark"—embody her faith in collective strength, echoing the group’s shared resolve. Through quiet persistence, she navigates the balance between self-doubt and acceptance, anchoring her evolution in both personal aspirations and the group’s intertwined destiny.