Mitsuru Kobayashi forms one half of the central twin duo, initially depicted as a popular and adept fighter at an all-boys high school. He masters kendo but faces academic struggles, particularly in history. This drives him to persuade his sister Megumu to switch identities and sit his exams, sparking the story's primary chain of events.

While disguised as Megumu at her all-girls school, Mitsuru encounters Shino Takenaka, a deaf student bullied by Azusa Tokugawa. He intervenes to shield Shino, developing immediate and intense feelings for her. His protective instincts manifest fully as he actively defends Shino from Azusa's aggression. However, he steps aside upon realizing Shino's affections rest elsewhere, abandoning his pursuit.

Mitsuru's focus then shifts to Azusa Tokugawa, the initial antagonist. He uncovers her deep-seated vulnerability, born from a troubled family life where she felt unloved due to her father's preference for Shino and her mother. His feelings for Azusa transform as he comprehends her pain. This culminates in a bold proposal where he seeks to end her suffering, pledging to provide all the love denied her in childhood.

His character undergoes substantial evolution, transitioning from a self-centered teenager preoccupied with evading academic consequences and casual flirtations to someone capable of profound empathy and commitment. His relationship with Azusa deepens seriously, leading to marriage. The narrative concludes with them raising twins, a boy and a girl. Some interpretations note Azusa may have initially represented a rebound following Shino, and his early actions toward Azusa stemmed more from personal desire than complete understanding of her situation.