The models are from Illusion games Honey Select 1. There's a big community of making custom characters, backgrounds and animations.
You can get it on steam if you want to support them but it's pricey and limited unless heavily modded, You can also get a free version from 'Screwthisnoise' as it's what most of the community use. Just google Illusion and screwthisnoise. Better re-pack stuff is what you'd be looking for.
As for the coding and game engine i'm fairly vertain it's R'enpy but don't quote me on that i'm not 100% there.