Miki Koishikawa is the main protagonist of the series. She is a lively high school student at Toryo High School, where she is a member of the tennis team. Her ordinary life is turned upside down when her parents announce they are divorcing in order to swap spouses with another couple they met on a trip to Hawaii. As a result, Miki moves into a large house with both of her parents and the other couple, as well as their teenage son, Yuu Matsuura. Although initially resistant to the arrangement, Miki gradually comes to accept her new blended family.

Miki has a cheerful, friendly, and good-hearted nature. She is outspoken and supportive of those she cares about, and she tends to be emotionally expressive. At the same time, she can be insecure, whiny, clingy, and emotionally dependent on others. She is prone to jealousy and sometimes acts impulsively or stubbornly. She often worries about her appearance, seeing herself as average-looking compared to the more glamorous girls who surround Yuu. Her emotions are close to the surface, and she does not hide her feelings easily.

Miki’s primary motivation is to find love and stability amid the upheaval in her family and romantic life. She works part-time at an ice cream parlor called Bobson’s, and later at a small decoration shop, showing a desire for independence. She deeply wants her relationship with Yuu to succeed, but she also struggles with insecurity and the attention he receives from others.

In the story, Miki serves as the emotional center. Her evolving feelings for Yuu drive much of the plot. She initially reacts with shock and frustration to the parent swap, but over time she and Yuu grow close. They confess their love for each other and begin dating, though they keep the relationship secret from their parents. Their romance faces many trials: Yuu’s departure to study architecture in New York leads Miki to break up with him out of exhaustion from the distance, but they reunite when he returns for summer break. A later misunderstanding makes Yuu believe they might be half-siblings, causing a temporary breakup, but the truth is eventually revealed, and they decide to marry, receiving their parents’ blessing.

Miki’s key relationships shape her development. Her best friend, Meiko Akizuki, is a mature and wealthy classmate whom Miki often turns to for advice. Ginta Suou is a longtime male friend and her former crush from junior high; their past rejection left Miki heartbroken, but they remain friends, and Ginta’s lingering feelings add complexity. Arimi Suzuki, Yuu’s ex-girlfriend, tries to win him back, which fuels Miki’s jealousy and forces her to confront her insecurities. Through these interactions, Miki learns to navigate trust, jealousy, and her own emotional needs.

Miki’s growth is significant. She starts as a naive and somewhat dependent girl who is overwhelmed by the sudden changes in her life. As she faces romantic ups and downs, she becomes more self-assured and capable of making difficult decisions. She learns to stand by her choices and to communicate more openly, especially with Yuu. Her emotional openness is a trait that Yuu admires, as he is more reserved.

Notable abilities include her participation in the tennis team, where she is an active player. Her strength lies in her emotional honesty and her capacity to connect with others, even when conflicts arise. She is not particularly athletic or academically gifted, but her warmth and persistence define her. By the end of the series, Miki has matured into a young woman ready to take on the future alongside Yuu.