Hinako Yaotose is a high school student living alone in a tranquil seaside town, her life shaped by a childhood accident that claimed her entire family. This tragedy left extensive burn scars on her right shoulder and arm, which she habitually conceals under long sleeves. The profound trauma instilled a persistent sense of detachment from peers during summer and a passive desire for death, though she cannot actively end her life. She harbors a wish to be consumed by a powerful, sublime force like the ocean, mirroring her family's fate.

Her existence shifts upon encountering Shiori Ōmi, a mermaid emerging from the sea. Shiori reveals that Hinako possesses uniquely desirable flesh attracting dangerous monsters and proposes a pact: she will protect Hinako until the time comes to consume her. Hinako accepts, viewing Shiori as a potential fulfillment of her death wish while recognizing in the mermaid a paradoxical blend of cruelty and kindness akin to the ocean. This initiates a complex relationship where Hinako relies on Shiori for safety against supernatural threats.

Before Shiori, Hinako's only close connection was Miko Yashiro, a friend supporting her after the accident. Unbeknownst to Hinako, Miko is a fox spirit who watched over her for years, using manipulated memories to maintain her presence. Hinako experiences profound betrayal upon discovering Miko's true nature and the deception behind their friendship. This strains their bond, culminating in a critical moment where Miko, fighting her own urge to consume Hinako, bites off her own tails to regain control, demonstrating deep, sacrificial commitment despite the lies.

Hinako exhibits a calm, introverted, and logical personality, often appearing emotionally stagnant or isolated. Interactions with Shiori foster subtle changes. Experiences like attending a summer festival—a trigger due to its association with her family's death—where she holds Shiori's hand, or a school trip where she consciously attempts to act happier, begin to thaw her reserve. A pivotal moment occurs when she feels Shiori's heartbeat during a protective embrace, leading to a realization of Shiori's genuine fear of losing her and the emotional weight of their blood oath. This sparks a significant realization regarding her previous assumptions about Shiori's alien morality and the true cost of their pact, prompting regret and a shift towards valuing their connection and life.

Physiologically, Hinako possesses "supernaturally delicious and nutritious" flesh that draws monsters, attributed to Shiori's blood given during their initial childhood encounter. This blood granted an accelerated healing factor, explaining her survival from severe injuries in the family accident. Paradoxically, her flesh tastes revolting specifically to Shiori. Hinako displays unusual tolerance towards monstrous forms, evident from her childhood approach to the injured, monstrous Shiori with curiosity and shared food, and later comments showing she doesn't perceive Shiori or Miko as inherently terrifying despite their true natures.