As with all classically based drama the screenplay should have a protagonist (hero) and antagonist (villain) but these need not be as simply defined as goody and baddie. The important point is that all of your characters should be interesting to your audience.
THE PROTAGONIST – the person your story is about, the one who leads the action and who changes the most during the story.
THE ANTAGONIST – this is the person who will provide the main opposition to your lead character.
SECONDARY CHARACTERS – the protagonist and antagonist will both have friends and allies who will aid them, these can be family, friends, a dog or even a robot. The relationship between the leading characters and the secondary can often reveal a lot about them.