Forget the underdog, I love the antagonist in a story. The antagonist is actually the most important character.Your plot is driven by the main character’s interactions with all the antagonistic elementsin the story. You can have a great, well-developed hero (I am using the male form of hero only for naming purposes but you can substitute the word that fits your character best: heroine, pet, inanimate object, or anyother form your main character takes). Without someone for your main character to go up against, your story will never hold your readers’ interest. Nobody wants to read a book about a great person that nothing happens to and who has a wonderful life. It isn’t interesting, it isn’t realistic, and it certainly is not relatable.
Think about your own life: do you remember all the uneventful days that you get up, go to work, come home, and are completely uneventful? Are those the stories you tell around the water cooler or on a night out with friends? Or are you reliving that time you stood up to your boss, fixed something that seemed impossible to sort out, or took a chance and met somebody new? Which story would you rather hear from someone else—an average day in their lives or that one time when they did something incredible? I know which one I’d pick. Since you’re trying to get readers to hang in there with you from the beginning to the end of your story, you should know which one is the correct choice, too.
Your hero needs a struggle. Struggling is a fact of life regardless of who you are, and it can take many different forms. It can be a physical struggle, like climbing Mount Everest, or it can be an emotional struggle, like coping with a loss. Your hero needs to change over the course of the story, discovering something about themselves that readers can identify with along the way. Something needs to happen to him, and that something is usually introduced by the antagonist.
Not every antagonist is an evil villain. They can be regular people who simply challenge the hero to be something more, something better. Some antagonists are not even people—it can be anything from an element of nature to a physical object or even a concept. Anything can be used to pit against your main character.
An antagonist needs to be nearly as well, if not as well, fleshed out as the main character in order to be a believable foe. There is no suspense, no page-turning compulsion if the adversary is not worthy of your hero. Their motives can be revealed over time or at a single point along the way, but they need some kind of justification for going up against the main character in your tale. As a way to move your plot forward, they deserve the same focus and dedication you spend on the other elements of your story. Your readers will thank you!