A story, novel or play may achieve the status of literature for various reasons: a distinctive writing style, a gripping plot or noteworthy philosophical insights. Above all else, however, works of literature have strong, memorable characters. Though great characters feel completely original, studying their function in the story allows critics to categorize them into a few basic character types.
The protagonist is the hero or heroine of a story. A traditional protagonist has a goal, something she wants. She may not achieve this goal, but by pursuing it, she moves the plot forward. The protagonist undergoes character development during the course of the story. She is not precisely the same person at the end as she was at the beginning, as a direct result of the story's events. One famous protagonist is Odysseus, hero of Homer's "Odyssey," who has a series of adventures at sea while pursuing the goal of returning to his home and family.
The antagonist stands in opposition to the protagonist's goals. An antagonist does not need to be evil or even malicious, but he needs to be a suitable impediment for the protagonist in order to generate high stakes of conflict within the story. Nurse Ratched, in the novel "One Flew Over the Cuckoo's Nest" by Ken Kesey, is the antagonist to the institutionalized hero, Randle Patrick McMurphy. Her values of order and tranquillity within the asylum are in conflict with his values of liberty and individual choice.
The foil is the protagonist's counterpart, who helps the story generate additional dramatic or comic tension. While the antagonist is directly opposed to the protagonist's aims, the foil serves as more of a contrast to the protagonist's personality. Often, the foil brings out the protagonist's lighter side, or causes the protagonist to think about the story's problems in a new way. The classic example is Falstaff from Shakespeare's "Henry IV," who is the outsized, buffoonish foil for the heroic Prince Hal.
Most stories and novels have roles for minor characters, which can be quickly characterized and made familiar to the audiences of their era. These characters do not develop dramatically to the extent the protagonist does, but perform more limited roles to help advance the story. The greedy banker, the stern principal and the nerdy computer programmer are all instantly recognizable to modern American readers. If the writer is skilled enough, she can take a stock character and show original dimensions to their personality.