A protagonist is a leading character in a literary work. A protagonist is often, but not always, the hero of a story. An antagonist is the opponent of a protagonist.

Protagonist comes from the Greek word prōtagōnistēs, from prōt (“first”) and agōnistēs (“competitor”). 

According to legend, Thespis was the first ancient Greek playwright to introduce a protagonist. Before, plays had consisted only of choruses, which recited poetry praising Dionysus (the god of theater, among other things). Still, Thespis added a character actor to his plays. Eventually, Greek drama expanded to encompass three characters—a protagonist, a deuteragonist, and a tritagonist. 

An example of a protagonist is Eliza Sommers in Isabel Allende’s novel, Daughter of Fortune.

see: antagonist