Explanation:

- Opponent: Typically refers to someone who is against you in a conflict or competition.

- Option 1: Rival: Another contender or competitor, often with similar abilities, challenging one's successes.

- Option 2: Antagonist: A person actively opposing or hostile to someone, often seen in storytelling.

- Option 3: Mischievous: Means playfully causing trouble; not directly opposing, but more playful in nature.

- Option 4: Ally: A person or group that is on your side, offering support or working together toward a common goal.

- Answer: Ally is indeed the nearest antonym to opponent, as an ally supports whereas an opponent challenges.