1
companion;partner;company
あなたのダンスの相手はだれですか。
Who is your dance partner?
2
other party;addressee;counterparty
あいつは海千山千だから、取り引き相手としてはかなり手強いぞ。
All those tricks he's got up his sleeve make him a formidable client.
3
opponent (sports, etc.)
そのボクサーは第１ラウンドで相手を倒すつもりだと言った。
The boxer said he would knock down his opponent in the first round.