Villians and supporting characters may require a bit more work on your part than just using fandom characters. The concerns for them are similar, but are more important when considering villains, so I’m going to focus on them first. I’ll talk about supporting characters tomorrow.
The first thing to consider when creating a villain, is that you villain isn’t limited to a supervillian Darth Vader/Joker/Lex Luther type. Villain isn’t even a term limited to a criminal. A villain could simply be the other leg in a love triangle, or a menacing boss making everyone’s lives more difficult. Basically, a villain is whoever is keeping your hero from achieving their goal.
The second thing consider is that even villains need to be credible and realistic. The villain should be just as fleshed out and developed as the hero(es), because your hero(es) needs someone worthy to go up against. It helps keep the stakes high, and gives the reader another reason to care about your work.