Origin and history of protagonist

late 14c., agonie, "mental suffering" (especially that of Christ in the Garden of Gethsemane), from Old French agonie, agoine "anguish, terror, death agony" (14c.), and directly from Late Latin agonia, from Greek agōnia "a struggle for victory" (in wrestling, etc.), in a general sense "exercise, gymnastics;" also of mental struggles, "agony, anguish."

This is from agōn "assembly, mass of people brought together," especially to watch the games, hence, "a contest," then, generally, "any struggle or trial;" from the verb agein "put in motion, move" (here specifically as "assemble, bring together"), which is reconstructed to be from PIE root *ag- "to drive, draw out or forth, move."

In English, specifically of the struggle that precedes natural death (mortal agony) from 1540s. The sense development perhaps involves "pain so severe as to cause struggling." The sense of "extreme bodily suffering" is recorded by c. 1600.

I like a look of Agony,

Because I know it's true

[Emily Dickinson]

"one who contends with another," 1590s, from French antagoniste (16c.) or directly from Late Latin antagonista, from Greek antagonistēs "competitor, opponent, rival," agent noun from antagonizesthai "to struggle against, oppose, be a rival," from anti "against" (see anti-) + agonizesthai "to contend for a prize," from agon "a struggle, a contest" (see agony). Originally in battle or sport, extended 1620s to any sphere of human activity.