Jin Koishikawa is the father of the series' protagonist, Miki Koishikawa. He works as an accountant at a bank. In terms of his background, he was once in a relationship with Chiyako, who would later become his second wife after the series of partner swaps. He eventually married Rumi after helping her recover from her own breakup with Youji Matsuura. The couple moved to England, where their daughter Miki was born, before returning to Japan.

Jin is characterized as having a somewhat quirky or carefree personality. He is known to be a little childlike, a trait he shares with his ex-wife Rumi. Despite his age, he maintains a playful and warm demeanor, especially in his interactions with his family. He is depicted as being somewhat out of touch in a lovable way, but his deep affection for his daughter, Miki, is a central aspect of his character.

His primary motivation stems from a desire for personal happiness and the well-being of his family, even if his methods are unconventional. This is most evident when he and his wife Rumi decide to divorce and swap partners with the Matsuura family. This decision, which forms the catalyst for the story, is not portrayed as malicious but rather as a mutual agreement between two couples seeking a more fulfilling emotional connection. His role in the narrative is that of a supporting parent who, along with the other adults, creates the unusual living situation that drives the plot forward.

Jin's key relationships are primarily with his family members. His relationship with his ex-wife Rumi remains amicable, as they continue to co-parent and live together under the same roof after their respective remarriages. He marries Chiyako Matsuura, becoming the stepfather to her son, Yuu Matsuura. The dynamic between Jin and his stepson is positive, and they are shown to have a genuine father-son bond, such as when Yuu playfully teases Jin for his lack of skill at video games. His relationship with his biological daughter, Miki, is loving, though they are not seen interacting as frequently as she does with her stepfather, Youji Matsuura.

Throughout the story, Jin does not undergo significant personal development, as the focus remains on the younger characters. However, his willingness to embrace the unconventional family structure demonstrates an open-minded and adaptable nature. He has no notable abilities or special skills highlighted in the series beyond his professional role as an accountant and his function as a supportive parent.