Now that you’ve thought about why you love them, think about what you dislike about them. What do they do that frustrates you? What flaws do they have?
Fully fleshed-out characters have good and bad traits. No one is all one thing. Focus on your favorite character’s bad traits. Now write them down.
If you’re struggling to find a bad trait, think about a trait that another character in the story wouldn’t like and write about that.