When writing a story most writers begin with creating the protagonist. Of course we all know that stories are about characters wanting something and having obstacles that stand in the way of he or she reaching their goals. Aside from creating a compelling protagonist, creating the antagonist is just as important. The antagonist stirs things up and creates problems.
The antagonist is often the opposite of the protagonist. Good versus evil. Weak verses strong. Popular girl verses nerd. Werewolf verses vampires. Having characters that are opposite creates conflict. Conflict makes things interesting. After you create your protagonist develop a character who has the opposing traits of the protagonist.