Add your avatar to the game.

https://atomrpg.com/cdn/Avatar2_0.zip

Now you need to change the avatar, go to(or create) mod directory Mods (%AppData%/../LocalLow/AtomTeam/Atom/) and change or add your files.

PNG - avatar image
ASSET - avatar configuration file, appearance style.

All file names must be in format [ava1-ava39]

Mod source: https://github.com/atomrpg/modkit/tree/avatar

Also option to add via bundle using Unity (it is important to set FILE_SYSTEM_LOAD = false if using bundle)