“Protagonist: (Greek ‘first combatant’) The first actor in a play; thence the principal actor or character. In Greek tragedy the playwright was limited to the protagonist (first actor), deuteragonist (second actor) and tritagonist (third actor). It is probable that in the first place Greek drama consisted of a Chorus and the leader of the Chorus. Thespis (6th century BC) is believed to have added the first actor to give greater variety to the dialogue and action. The second and third were added by Aeschylus and Sophocles respectively. The protagonist has come to be the equivalent of the hero.”
Excerpted from: Cuddon, J.A. The Penguin Dictionary of Literary Terms and Literary Theory. New York: Penguin, 1992.