What is the X in X-mas?
1670s, "principal character in a story, drama, etc.," from Greek protagonistes "actor who plays the chief or first part," from protos "first" (see proto-) + agonistes "actor, competitor," from agon "contest" (see act (n.)). Meaning "leading person in any cause or contest" is from 1889. Mistaken sense of "advocate, supporter" (1935) is from misreading of Greek protos as Latin pro- "for."
in ancient Greek drama, the first or leading actor. The poet Thespis is credited with having invented tragedy when he introduced this first actor into Greek drama, which formerly consisted only of choric dancing and recitation. The protagonist stood opposite the chorus and engaged in an interchange of questions and answers. According to Aristotle in his Poetics, Aeschylus brought in a second actor, or deuteragonist, and presented the first dialogue between two characters. Aeschylus' younger rival, Sophocles, then added a third actor, the tritagonist, and was able to write more complex, more natural dialogue. That there were only three actors did not limit the number of characters to three because one actor would play more than one character