Side characters might not be the main protagonist of the story but they still have their own motivations and desires, so you want to make them well-rounded. However, whether it’s the jerk friend or the annoying sidekick, well-rounded doesn’t have to equate to likeable. The balance is making them seem human versus a stereotype. One of the issues I had with my side character Britta in BURN OUT is that she was way too unlikeable in early drafts. My beta readers disliked her so much that it caused problems with how the end of the book played out.
The challenge I had was to make the readers feel a little more sympathetic toward her even though she wasn’t the nicest character (okay, she was a total bee-atch). This involved more explanation of her character and what made her the way she is, which translated into more scenes throughout the book. One of my best beta readers read the last draft and said she couldn’t believe I’d made her care about Britta, but the thing is, I cared about her too by that last rewrite. I don’t think it’s a bad thing to get the first draft down knowing you will still have some fleshing out to do. Basically, if every character in your book was nice, it would be a very boring book. So make your characters mean, revengeful, or annoying—just keep them real.