Here's my highly speculative theory on possible secret characters. In the gameplay trailer we see the non-date locations in the Girl Finder. We have the bedroom in the middle (with Kyu) and a surrounding eight locations. Five of them are occupied by the ten girls (always in pairs) you're currently not on a date with. If we were to give the remaining two their own location, we're left with two locations. That's one for the two Nymphojinn, and a final one, just maybe, for two secret characters.

Yea I’ve thought about that! I figure dev wanted the map to be 9 different locations so it’s all even, but the spaces do make you wonder if it’ll be filled up at some point, we’ll have to wait and see but maybe the other two spots will get filled up by secret girls, assuming there are any, honestly, I kinda want a vampire girl lol

Thanks for the ask! ☺️☺️