Mayu is a central character in the story and the best friend of the protagonist, San. Originally a transfer student from Tokyo, Mayu comes from a distinguished family on the island of Okinawa. Physically, Mayu is tall and notably good-looking, a trait that draws admiration from classmates and contributes to a calm, composed presence. The name itself is written with the kanji for cocoon, a symbol that resonates deeply with the character's role and the story's themes.

At first glance, Mayu presents as a gentle and protective girl. As the war in the Pacific escalates and the situation becomes increasingly dire, Mayu emerges as a stabilizing force for San. Mayu's demeanor is soft-spoken and soothing, often casting a figurative spell to shield San from the most brutal realities of their surroundings. This protective instinct is Mayu's primary motivation: to preserve some semblance of safety and normalcy for San amid the chaos of being drafted as a nurse for wounded soldiers. Mayu encourages San to retreat into an imagined, sheltered mental space referred to as an imaginative cocoon, a shared fantasy that serves as a psychological defense against the hellish battlefield their island has become.

The most significant revelation about Mayu comes late in the narrative: Mayu is biologically male. This identity was hidden to avoid the wartime conscription of young men. Mayu lived and presented as a female student throughout the story, a secret held closely. This secret adds a deep layer of complexity to the character's personality. Mayu's quietness is not just serenity but also the caution of someone constantly guarding a life-threatening truth. The protective nature toward San is also intertwined with romantic feelings, a heartfelt attachment that Mayu carries without fully revealing the truth of the self until the final moments.

The relationship between Mayu and San is the emotional center of the story. Their bond evolves from schoolgirl friendship into a profound, co-dependent partnership. For San, Mayu is a guardian and an emotional anchor. For Mayu, San is the one person worth protecting above all else, even when San initially does not know the full nature of that protection. Other relationships with classmates are generally cordial, with Mayu being seen as a cool or princely figure among the group, but this distance is partly due to the necessary isolation the secret demands.

Mayu's character undergoes a tragic but necessary arc. The death occurs during an escape with San, struck down by gunfire from the sea. The moment of death is pivotal: it is only after Mayu falls that the physical truth of the character's male body is discovered by San. This loss shatters the metaphorical cocoon that Mayu had built around San, forcing San to confront reality directly and to grow beyond the protection Mayu provided. Mayu's final act is a confession of love, a crucial emotional release that allows the character to die authentically, having spoken the truth.

Mayu possesses no supernatural abilities. The notable traits are emotional resilience, physical strength unusual for a girl (hinting at the biological truth), and a powerful capacity for compassion and deception driven by the will to survive and protect. The character's greatest ability is the creation of a psychological sanctuary through storytelling and gentle reassurance, granting a momentary escape from the horror of war.