Mayu Akihime is the introverted twin sister of Saki Akehime, both distinguished by their English maternal heritage despite sharing twin status. Shy and reserved, Mayu contrasts sharply with Saki's outgoing popularity. She excels academically as a top student at school, positioning her as a potential tutor when Saki faces expulsion over failing grades. Secretly, Mayu harbors intense romantic fantasies about their teacher, Tsuguya-sensei.

When Saki's illicit affair with Tsuguya-sensei is exposed, blackmailers target both sisters. They exploit Mayu's unrequited feelings and academic reputation, coercing her into compliance through degrading acts—including group exploitation and public humiliation—to shield Saki and the teacher. The blackmail escalates to physical violations: devices filled with semen are inserted into her body, forcing her to endure their retention during school hours. This inflicts severe physical pain and psychological torment, deepening her isolation due to her innate introversion. Her narrative role centers on victimhood, with antagonists systematically exploiting her vulnerabilities.