Miyako Yamashina attends Yanagihara High School alongside classmate Takashi Haneda. Haunted by childhood abandonment from her mother, she carries deep loneliness and struggles to form connections. She works the counter at her family's tobacco shop, routinely watching Takashi pass by.

Initially withdrawn and solitary, Miyako reveals a dual nature—one side quiet and reserved, the other strikingly extroverted and expressive, especially around Takashi. After gathering courage to speak to him, her demeanor shifts dramatically in his presence. She becomes openly affectionate, engages in relentless conversation, and makes suggestive remarks, indifferent to onlookers.

Her romantic past includes an ex-boyfriend whose rejection triggered extreme anguish. Miyako covertly entered his apartment using kept keys to sleep in his bed, actions he discovered via security footage. Publicly confronted by him during a date with Takashi, she unleashed an emotional breakdown expressing suicidal thoughts.

In Takashi's narrative path, Miyako evolves into a possessive partner, anxious during separations and flooding him with messages if he delays replies. Learning of his dissociative identity disorder, she references similar personal struggles, identifies his alters as "inner self-helpers," and urges therapy. They progress to cohabitation while Takashi seeks work and psychiatric treatment.

An alternate narrative conclusion depicts Miyako spiraling into violent obsession culminating in Takashi's death. Supplementary prequel scenarios further dissect her compulsive pattern of breaking into her ex-boyfriend's residence.

Beyond these intense relationships, Miyako engages in recreational online gaming, indicating wider behavioral facets.