Fuka Hasegawa is a second-year student at Yukishina Junior High School who becomes the sixth participant to arrive at the mysterious castle. She is a talented pianist, a skill that defines much of her life and her struggles. Raised solely by her single mother, Fuka has no father figure in her life. Her mother is intensely dedicated to Fuka's musical career, investing all of the family's limited financial resources into piano lessons and studio equipment. This financial pressure comes at such a great cost that the household frequently lacks basic utilities like water, heat, and electricity.

The pressure on Fuka is immense, as she feels her entire value as a person rests on her success as a pianist. Her mother, who may be projecting her own unfulfilled artistic ambitions, pushes Fuka to dedicate all her time to practice at the expense of her schooling and social life. The situation worsens when Fuka underperforms in competitions, failing to advance past early rounds. Her mother perceives these outcomes as failures, which deepens the emotional strain and neglect at home. Consequently, Fuka lives under crushing stress and heartbreak, believing that being good at piano is the only thing she has to offer the world.

Fuka has a distinctive appearance, wearing glasses and often a stoic expression. Her personality is blunt, cool-headed, and pragmatic, which leads her to serve as the voice of reason within the group of castle visitors. Within a Freudian trio comprising herself, Kokoro, and Aki, Fuka represents the "Superego," providing balance during conflicts and acting as a grounding presence. Initially, she is reserved and not a believer in love, presenting a somewhat icy exterior. However, this outward stoicism is a thin veil for her deep emotional struggles. When her fellow castle members Ureshino, Aki, and Kokoro give her birthday presents and flowers, Fuka is genuinely touched, causing her to warm up considerably and begin defrosting from her initially cold demeanor.

The castle serves as a vital refuge for Fuka, offering a temporary escape from the unsustainable expectations of her home life. Within its walls, she has the opportunity to exist beyond her identity as a pianist and discovers friendship and self-worth that are not tied to musical performance. Through her relationships with the other children, she begins to seek validation and support outside of her mother's demanding framework. Her personal journey within the story reflects a shift toward finding value in herself as a person, rather than solely as a performer, even if her difficult family situation remains largely unresolved.