Fūka Hasegawa, a second-year Yukishina Junior High student from 2018, arrives at the castle as the sixth participant. A talented pianist, Fūka's musical gift brings intense pressure fueled by her single mother's significant financial and emotional sacrifices. Her mother pours all resources into piano lessons and studio equipment, leading to severe household neglect where basics like water, heat, and electricity are frequently absent. This neglect intensifies when Fūka underperforms in competitions, perceived by her mother as failure. Consequently, Fūka feels her value exists only through her piano skills, a belief causing crushing stress and heartbreak.

Within the group, Fūka presents a blunt and cool-headed personality, often acting as the voice of reason. She assumes the role of the "Superego" in a Freudian Trio with Kokoro and Aki, providing balance during conflicts. Though initially reserved, she gradually connects with the other castle members, forming meaningful bonds. This outward pragmatism thinly veils the deep emotional struggles she carries from her real-world burdens.

The castle offers Fūka temporary refuge from unsustainable expectations, a space to exist beyond her pianist identity. Here, she discovers friendships and self-worth untethered to musical performance. While her family situation remains unresolved, her participation signifies a shift towards seeking validation and support outside her mother's demanding framework.