Explanation:

- Opponent: Someone who competes against or fights another.

- Option 1: Rival - Another contestant or competitor, similar in meaning to opponent.

- Option 2: Antagonist - Someone who actively opposes or is hostile to another, similar in meaning to opponent.

- Option 3: Faithful - Refers to being loyal or devoted, but does not directly contrast with opponent.

- Option 4: Ally - A person or group that cooperates with another; directly contrasts with opponent.