1665–75; < Greekprōtagōnistḗs actor who plays the first part, literally, first combatant, equivalent to prôt(os) first + agōnistḗs one who contends for a prize, combatant, actor. See proto-, antagonist
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."