Haruna Wakakusa is the central character and one of the protagonists of the story. She is a high school girl living in a Tokyo suburb with her mother during the 1990s. Haruna is characterized as a straightforward, unrelenting, and nonconformist teenager. She exhibits a proactive nature, often taking action where others remain passive, which is first demonstrated when she intervenes to stop her boyfriend, Kannonzaki, from bullying her classmate, Ichiro Yamada. This act of saving Yamada is the catalyst for their unlikely friendship.

Despite her assertive exterior, Haruna navigates a deep sense of emotional emptiness and detachment. Her motivations are complex and not always clear, even to the reader. She continues a relationship with her boyfriend, Kannonzaki, despite seemingly hating his bullying nature and not enjoying his company, raising questions about her internal state. This contradictory behavior has been interpreted as a possible result of her parents' divorce, suggesting she may feel lonely and cling to an insipid relationship out of a fear of ending up like her parents, or as a way to keep a check on her boyfriend's bullying of her new friend. She presents a plucky façade to the world, but internally she feels a profound sense of nothingness.

Haruna serves as the primary point of view character and the connective thread that links the other major figures in the narrative. Through her friendship with Ichiro Yamada, she is introduced to Kozue Yoshikawa, a popular student and professional model. Together, these three emotionally damaged teenagers form an intense, albeit brief, bond over the course of a single semester. Yamada and Kozue both confide in Haruna, trusting her with their most intimate and unsettling secrets, including Yamada's discovery of a corpse by the river, a secret she accepts without judgment. Her relationships are fraught with difficulty. Her boyfriend, Kannonzaki, is hotheaded and rash, and he continues to bully Ichiro despite her objections. Her best friend, Rumi, becomes entangled with Kannonzaki on the side, further complicating the social dynamics. Haruna's choices, particularly her decision to befriend and protect Yamada, set off a chain of events that have far-reaching and disastrous consequences for everyone involved, demonstrating that even a well-intentioned individual can be an instigator of tragedy.

Throughout the story, Haruna begins to question the life she has led up to that point, marking her primary area of development. She is forced to confront the cruelty of her peers, the vapid nature of her romantic relationship, and the desperate search for identity that defines her teenage existence. Her growth is not about gaining special skills but about a painful emotional awakening. She has no notable superhuman or extraordinary abilities; her defining traits are her straightforwardness, her capacity for loyalty, and her role as a quiet observer whose actions inadvertently drive the plot toward its dark and explosive conclusion.