Jin Koishikawa is the father of the protagonist, Miki Koishikawa. He works as an accountant at a bank, a profession that suggests a stable and detail-oriented career, though his personality is often described with contrasting traits. While he holds a responsible job, he is also depicted as being somewhat childlike and scatterbrained, particularly within the family home. Despite this absent-mindedness, he is a caring and affectionate parent who dotes on his daughter.

His personal history is deeply interconnected with the central, unconventional premise of the story. Years before the series begins, Jin was in a relationship with a woman named Chiyako. After their breakup, he helped his friend Rumi recover from her own separation from Youji Matsuura. Jin and Rumi eventually married and moved to England, where their daughter Miki was born, before the family later returned to Japan. This established history is upended when Jin and Rumi re-encounter Chiyako and Youji. In an unusual agreement, the two couples decide to divorce and swap partners; Jin leaves Rumi to marry Chiyako, while Rumi marries Youji.

Following this decision, Jin becomes the stepfather to Yuu Matsuura, Miki's love interest. His role in the story is primarily defined by his participation in this unconventional family structure. As the story progresses, he lives with his new wife, Chiyako, and his daughter Miki, while maintaining a friendly relationship with his ex-wife Rumi and her new husband. Despite living under the same roof, Miki is not seen interacting with her father as frequently as she does with her stepfather, Youji.

His key relationships are all defined by this swap. His bond with his daughter Miki is affectionate, though she often finds herself exasperated by her parents' unusual choices. His new marriage to Chiyako appears to be a happy one, and the four parents generally remain on good terms, having reached an amicable agreement about their living situation. As an adult figure, Jin does not possess any notable abilities relevant to the dramatic or athletic conflicts faced by the teenage protagonists; his primary narrative function is to be one of the four parents whose actions set the entire plot in motion. His personal development is minimal, as the focus of the series remains on the younger generation; he largely remains a static, albeit loving and slightly eccentric, parental figure throughout the story.