Yup, a lot of games on here are made with Koikatsu assets. Koikatsu is like a hentai unity engine more than an actual game itself though. Its purpose is for developers to make games out of them, like this one.
That interesting. I've always used Koikatsu and Honey Select for strictly animations. Never have I used it for a Ren'Py.... I have some frozen character models I never used... I might make a Ren'Py.
