Hanako Yamadagi was born in Vienna, Austria, to Japanese parents who toured globally as musicians. Her international upbringing involved frequent parental travels, culminating in the family's return to Japan. Discovering idol music in her mother's collection sparked her deep admiration—nothing shone brighter to her than idols. Despite possessing idol-like qualities including dance skills, she remained fundamentally tone-deaf, resulting in exceptionally poor singing that often drew negative reactions when she attempted it.

Initially, she managed the struggling idol unit Ongaku Shōjo at Pine Records, handling logistical tasks like outfit planning and group dynamics mediation. Her cheerful optimism and reverence for idol work drove her behind-the-scenes dedication. She constantly carried "DoruDoru the Idol Monster," a handmade plush she once believed represented real idols before unit members corrected her.

Her interactions with Ongaku Shōjo members radiated enthusiasm and affection. She frequently praised Uori Mukae's voice and figure, held Uori's hand to advocate for her cooking, and shared hugs in onsen settings. She blushed when Kiri Mukae leaned on her, holding Kiri's hand during baths while discussing Uori. She called Hiyo Yukino "very cute" and found Shupe Gushiken "blindingly beautiful" after makeup, reacting with shy panic during nudity—like covering Shupe's body or encountering Miku Nishio at the onsen.

In Episode 12, Hanako transitioned from manager to full Ongaku Shōjo member. She performed the group song "Shining Piece" and debuted her solo "Idol☆Icchokusen." Though she received no character song single, her vocals featured in group albums including directed and non-directed song versions. This fulfilled her core aspiration: to stand alongside the "shiny" Music Girls she admired and root for their success, overcoming her initial limitations to become an idol.