So...
I've sucessfully added a mercenary unit in my mod. Everything works fine, except that the unit info image (the one that shows in the pop up info of the unit, with stats and description), and the unit card image are not showing. Instead, a generic medieval peasant is there.
I've found out that I did not have these images inside the UI folder for the specific faction, but even after adding it, it still showd the generic peasant.