antagonist (n.)

"one who contends with another," 1590s, from French antagoniste (16c.) or directly from Late Latin antagonista, from Greek antagonistes "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.