noun
opponent; adversary; dissenter
Refers to a person who opposes a plan, idea, or another person in a debate, vote, or conflict. Neutral term; can be used in formal and everyday contexts.
その計画には反対者が多かった。
There were many opponents to the plan.
彼は議論の中で反対者の意見をよく
He listened carefully to the dissenter's opinion during the debate.
相手 is a neutral term for 'partner' or 'opponent' in a game, match, or interaction, without the connotation of dissent that 反対者 carries.
Compound of 反対 (opposition) + 者 (person). The reading is standard on'yomi.