An opponent is someone who is in OPPOSITION to another person or group, often in a competitive situation. OPPOSER (7 letters) is someone who opposes or resists something, ATTACKER (8 letters) is someone who aggressively assaults or attacks, CONTESTANT (10 letters) is someone who takes part in a competition, CANDIDATE (9 letters) is someone who is nominated for an office or position, ASSAILANT (9 letters) is someone who makes a violent attack, OPPOSITION (10 letters) is the act of opposing or resisting something, FIGHTER (7 letters) is someone who fights or is prepared to fight, OBJECTOR (8 letters) is someone who expresses strong disapproval or disagreement, CHALLENGER (10 letters) is someone who issues a challenge, COMPETITOR (10 letters) is someone who competes with another for the same objective, ADVERSARY (9 letters) is someone who is opposed to another in a contest, ANTAGONIST (10 letters) is someone who actively opposes or is hostile to another, FOE (3 letters) is an ENEMY or opponent, ANTI (4 letters) is someone who is opposed to a particular policy or practice, ENEMY (5 letters) is someone who is actively opposed or hostile to another, RIVAL (5 letters) is someone who competes with another for the same objective, and FIEND (5 letters) is a very wicked or cruel person.