Miki Koishikawa is the central protagonist of the romantic comedy-drama series. She begins the story as a lively and cheerful high school student attending Toryo High School, where she is an active member of the tennis team. Her personality is a blend of outgoing, friendly, supportive, and outspoken traits, balanced by a more vulnerable side that includes insecurity, clinginess, and emotional dependence on those close to her. She can be short-tempered, jealous, gullible, and stubborn at times, but she is fundamentally good-hearted and deeply caring.

Miki’s life is turned upside down when her parents return from a trip to Hawaii and announce that they are divorcing in order to swap spouses with another couple, the Matsuuras. The two families decide to move into a large house together, and Miki finds herself living under the same roof as Yuu Matsuura, the other couple’s son. At first, she is resistant and skeptical about the arrangement, but as she gets to know Yuu, she begins to fall in love with him. Her motivations throughout the story are largely driven by her desire to understand her own feelings, navigate her complicated family situation, and build a genuine connection with Yuu despite the many obstacles that arise.

Her role in the story is that of the main viewpoint character; the narrative follows her emotional journey as she deals with her parents’ unusual relationship, her growing feelings for Yuu, and the romantic tensions created by her long-time friend Ginta Suou, on whom she had a crush in junior high, and Yuu’s former girlfriend Arimi Suzuki. Miki’s indecisiveness between Ginta and Yuu frustrates some characters, but it reflects her genuine confusion and fear of making the wrong choice. She works part-time at an ice cream parlor called Bobson’s and later at a small decoration shop, which gives her some independence and perspective.

Key relationships include her best friend Meiko Akizuki, a mature and wealthy girl who provides emotional support and advice. Her relationship with Yuu Matsuura is the emotional core of the series; they start as reluctant step-siblings, become friends, then lovers, and eventually overcome misunderstandings about their possible blood relation to marry. Her friendship with Ginta Suou is complicated by their shared history and unresolved feelings, but they eventually move on as Ginta develops a relationship with Arimi. Miki’s bond with her parents, Jin and Rumi, is also central, as she learns to accept their unconventional choices.

Over the course of the series, Miki undergoes significant development. She starts as a somewhat naive and emotionally dependent girl who struggles with jealousy and self-doubt, particularly when comparing herself to the more sophisticated girls who orbit Yuu. As she experiences heartbreak, time apart from Yuu, and the threat of being separated by a false belief that they are half-siblings, she grows more resilient and self-aware. By the end, she becomes more confident in her love for Yuu and more willing to stand up for what she wants, culminating in their marriage and the approval of both families.

Miki does not possess any supernatural or extraordinary abilities; her notable traits are her emotional openness and her capacity to express her feelings clearly, which Yuu admires because he struggles to do the same. She is also a decent tennis player, but her strength lies more in her social warmth and her ability to form deep emotional bonds. Her journey is one of learning to balance her emotional nature with the maturity needed to handle complex romantic and family dynamics.