An antagonist is a character in a story who opposes the protagonist. They often have traits that directly differ the protagonists and can be used to create tension and conflict in a story.
In tackling character development, the process is similar in all characters, positive or negative.
Antagonists can be human or non-human, such as animals, forces of nature, or even ideas. They are often portrayed as being selfish, power-hungry, or otherwise villainous. Writers need to create well-rounded antagonists with motivations and goals that readers can understand and relate to, as this helps make stories more interesting and engaging. The antagonist can start out seeming harmless, then revert to a truer nature, as can the protagonist. Take some risks, and mix it up. Readers like to be kept on their toes.