WMG / Piano no Mori

Ajino is Kai's father

Kai doesn't know who his father is. Kai's mother is a prostitute. Ajino seems to recognize Reiko (Kai's mother) at their first onscreen meeting. It's possible that Kai was the result of an unplanned pregnancy with Ajino. By the time she knew she was pregnant, there was no easy way to track down the father, since there were so many possibilities and brothels don't keep records of their clients. Years later, Ajino still remembers (or half-remembers) Reiko, but she's forgotten his face because she's had so many clients over the years.