A partner or buddy who regularly works or acts together with someone; an affectionate term for a partner or accomplice.
彼は長年の相棒で、どんな時も支えてくれる。
He is my longtime partner who supports me no matter what.
刑事ドラマで主人公と相棒が事件を追う。
In the detective drama the protagonist and his partner pursue the case.
盗みを働いた相棒は逮捕された。
The accomplice who committed the theft was arrested.