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.
Thanks, the information about AssetRipper is a possible solution I did not know about. But I’m afraid that my formulation led to confusion. I am aware that the background is “created” while talking with the NPCs. All I meant to say is that not having a background in custom character creation makes it strange to me that the given playable characters can’t be modified.
Still, thank you for trying to help with the explanation.
p.s. Please excuse if something is written in a strange manner, while it wasn’t mentioned in my previous comment, I’m not a native speaker.
Example of ripped Oriana's sprites.
I even have her in game description in the CharacterConfig.json. Which you can find in some compressed game files without decompressing.
Not sure what will happen if I talk to Oriana as Oriana. I'm betting it would break the AI, so I've been holding off since 1.1.10.