You can use the modding tools to change a character's portrait (directly in the mod folder) and their bio under Localization strings -- CharacterBios. It's probably easier to just change custom character bios and select their portrait in-game. However you cannot change their abilities so modding the default characters is the only way to use theirs.

How to edit the portaits (<mod id>\textures\ui\characters\):

Portrait chart (3rd image) : https://imgur.com/a/fIHkP

Each portrait is stored as a 512x512 tile in a .dds file. Paint .net handle that natively, other image editors like GIMP need a plugin.

All you have to do is copy a portrait, and paste it on the one you want to replace. You can configure a 512x512 grid and open as layer each file to make the process a lot faster.

Note that most portraits have two variations, with opened and closed eyes (except most NPCs, since they are not supposed to be playable). The "eyes closed" version is here for the characters to blink in-game, so make sure to replace each accordingly.

Source

Too bad you can't mod the backpack slots.

With Cheat Engine you can mod the backpack slots of each character. The change last the whole game too. http://fearlessrevolution.com/viewtopic.php?f=4&t=855&sid=c8db8b84fa016b1d688a0bba558cb45f