Fūka Hasegawa is a second-year middle school student who joins the other six children in the castle. She is recognizable by her glasses and a bowl-cut hairstyle, speaking with a notably high-pitched voice. Her background reveals a life shaped almost entirely by the pressure of musical achievement. Raised by a single mother, Fūka has been a dedicated piano student from a young age, to the point of being considered a prodigy. This pursuit, however, comes at a great cost. Her mother invests all of the family's limited funds into piano lessons and equipment, a financial strain so severe that their home sometimes lacks basic utilities like water, heat, and electricity. The driving force behind this sacrifice is her mother’s own unfulfilled ambition to succeed as a musician, a dream she now channels entirely into Fūka’s potential.

As a result, Fūka carries an immense burden, believing that her ability to play the piano is the only thing of value she has to offer. Her personality in the castle reflects this internal pressure; she is blunt, cool-headed, and often appears stoic. This reserved and slightly prickly exterior leads to her being seen as a kind of ice queen among the group. Her motivations are not driven by a desire for the castle's grand wish but are instead rooted in a deep-seated need to live up to the expectations placed upon her. When she begins to underperform in competitions, failing to advance past the first round, the feeling of having failed her mother despite all their sacrifices becomes a crushing reality, contributing to her stress and her withdrawal from regular school life.

Within the story, Fūka’s role is primarily as a member of the larger group of seven children who gather in the castle. She forms a particular dynamic with the other girls, notably standing up to defend the protagonist, Kokoro, and developing a close bond with the character Aki. Her key relationships evolve slowly. Initially aloof toward Ureshino, a boy in the castle who develops a crush on her, she is genuinely moved when he remembers her offhand mention of her birthday and gives her flowers, marking a significant step in her defrosting. This small act of kindness, along with receiving gifts from other friends like Aki and Kokoro, touches her deeply and helps her warm up to the group.

Fūka’s development throughout the story is a journey from isolation to connection, driven by the discovery that she has value beyond her musical talent. The friendships she builds in the castle offer a refuge from the pressures of her home life, teaching her that she can be accepted for who she is, not just for her ability to perform. While her notable ability is her high-level skill at playing the piano, the story ultimately reveals that this talent, which she saw as her only worth, is also a source of profound pain. Her personal growth lies in learning to see herself as a friend and an individual first, rather than solely as a prodigy whose success determines her family's well-being.