Jin told his daughter that she met Chiyako for the first time in Hawaii, but this was a lie to keep Miki unaware of his past. Jin dated Chiyako during college, but had a falling out because she was believed to have an affair with her employer. He left her and married Rumi unaware that Chiyako was pregnant with his child.

Koishikawa Jin is Miki's father. He is originally married to Rumi, but divorces her and marries Matsuura Chiyako, Yuu's mother. Jin is a playful character who loves to joke around. He loves his daughter, but doesn't focus on her needs as much as other fathers do. Like his daughter, Jin is athletic and was a member of the tennis and ski circle in college.