There are 2 other hidden girls, one you get by buying a goldfish and then dropping it in the trash at the park. The other girl you unlock by giving Kyu the dirty magazine and she will give you a strange object, take the object to the beach at night and attempt to give it to however is there then leave and the girl will appear.
Quelle: https://steamcommunity.com/app/339800/discussions/0/611698195160731471/?l=german