Wait for official support of custom character models. Trying to do this now will lead to your insanity.

To add new models now you'll need to integrate something like bepinex and write custom mod that will add a new race or replace existing. Or to decompile and rebuild the game with your models.

And a model must be modified so the game would be able to work with it, which is something that's really hard to do as HnS uses a specific way of combining meshes into one model.

Even just replacing a simple model in unity game without having access to source project is incredibly annoying and frustrating process, especially if this model is complex and animated.

If you want to try (don't) replacing in-game models, some general (and not working) steps are:

1. Edit your model so it will be as close to ingame's as possible (game uses heads, feet, breast, bodies, eyes, upper and lower teeth, tails, penises and decos like horns as parts of the body model, not one mesh and I'm not sure how they define physics enabled areas);
   
2. Install Unity of the same version game uses (as of 5.27 it's 2018.4.30 I think);
   
3. Create a scene and add all your meshes there and center/transform them properly;
   
4. Build your game;
   
5. Open your game's .assets files with UABE and find your meshes;
   
6. Export dump and raw data of the meshes through UABE;
   
7. Open .assets files of HnS with UABE and find meshes you want to swap (use Unity AssetStudio to find them faster);
   
8. Export their dump and raw data and save it as a backup.

Easy so far, right?

1. Now you need to rename dump files of your meshes same as game's are named, then open dump files and change internal names to what game meshes use (if you can, better to do this with raw data files, but it may require using a hex editor application like HxD). All this is to make the game see and understand new meshes (it likely won't understand sh*t).
   
   
2. After all is done, import your dump or raw (the ones you've changed) into game's .assets files using UABE.
   
   
3. Save resulting .assets files somewhere else and then replace game's files with them (backup everything before that)

UABE and AssetStudio are available on Github.

99.9(9)% that it won't work and game will either be crashing or totally glitchng in most lovecraftian ways because animators won't be able to find the model and its vertices and apply transforms.

So to keep your sanity, just wait for the devs to add this option officially.

p.s. I pity the one who will be implementing this system.

damn at this point i might as well just wait for official support too

media player is that unreasonable?

edit: oh wait wrong import