I’ll answer based on a novel I read recently. In Scarecrow by Michael Connelly the main villain of the story doesn’t undergo change and doesn’t have much of a backstory other than a flashback or two into his past and that is the villain of a pretty good thriller. The character is still fleshed out in his actions though. The way I think of it is the character should be well rounded in your mind even if they don’t get a high word count in the book. That helps them shine in the few words they do get.