We all put a lot of thought into our protagonist but there is another character who is just as important that we sometimes overlook, the antagonist. I've read far too many novels with a textbook bad guy that struggles to be even two dimensional. It ruins the book for me every time. The antagonist often drives the plot just as much as the hero, and has a profound effect on the hero. That makes them just as important~or close to it~as the protagonist.
If you want your novel to be believable and your readers to sympathize with the hero, you must make sure your antagonist is fully developed. Get to know your bad guy or gal just as well as you know your hero. What are her motives? What is his past? What makes her do the things she does? What does he want out of the situation and why? These are all things that will make him or her well rounded. Everyone loves to hate a great bad guy so put the work into him or her and make them worthy of both the story and it's protagonist.
There should be something redeemable or sympathetic about the antagonist, yes really. It can be tiny, so tiny most people wouldn't consider it something that makes them that way. Maybe they were abused as a child, or have strong beliefs that drive them to a goal they believe is just. Its not an excuse for why they are the way they are, it's simply part of them that makes them believable to the reader.
A strong antagonist makes your protagonist that much better. They're worth the time and effort even if they die in the end!
Check out this post by Writer's Digest on Crafting Your Villain.