Let's get something straight right off the bat: Your story is about your protagonist. That is, the protagonist is the star. By definition. Even if you have a very engaging and sympathetic antagonist, the reader identifies more with your protagonist’s struggle and desire. If that’s not the case, you have the wrong protagonist. That’s the first thing to keep in mind when dreaming up and/or depicting your antagonist: the protagonist is the star of the show. The antagonist’s purpose is to serve the author’s goals for the protagonist.