Mods

Mods (or Modifications), are player-made changes to the game. Currently, only mods that add original guns to the game are supported through the Steam Workshop, though adding support for character mods is planned for the future.

While any modification can be made (and has been made, in the form of character mods, for example) due to the open source nature of the game, currently only gun modifications can be uploaded to the Steam Workshop.

Forums mods
Mods from the Relic Hunters Forum!

Workshop mods
Guns have been added to the Workshop and they're planning to add characters to it.

Some examples from the workshop are:

Creation
For the singular character-based modification created, there are currently no explicit instructions or guides on achieving this, most likely due to the game not supporting them. However, concerning the supported gun-modifications, there is currently a specific guide dedicated to the creation of gun modifications and another guide meant to explain how to properly setup up the moddable-version of the game.

The process seems to be fairly simple, in it taking the form of creating a gun sprite-more often the not, editing an existing gun sprite and copying said sprite, turning it into it's own creation-defining it as an 'object' in Gamemaker Studio 2 And then, going into the events tab, defining it'sproperties, such as it's reload time, range and firing speed.