itch.io

There is a tool, called AssetRipper, that allows you to extract the the images used for all of the characters, you can then use those images to create your own custom character out of one of the included character's assets, but you'll have to cut out each directional sprite, and put your own description in for the new custom character. For the background, it is stated for visual aspects only, but you can add some background info if it's important, like saying that your character is wearing something from a specific place or is a variant of their race/species that is commonly found in a certain region or speaks a certain language/ has a certain accent. Typically you give the characters your background as you play, they will repeatedly try to get you to answer questions about yourself, so you could talk about how you're an escaped convict or the chosen one or whatever, the game has a lot of room for you to make your own past. As for making .glb files, I've found that Trellis gave me the most usable results.