仇人 (chóu rén): 敵

He used to be my best friend, but since that argument, we have become enemies.

Those two families have been enemies for generations because of a dispute over water, and they still do not associate with each other.

With the police mediation, the two enemies finally sat down and had a calm talk.

He never dreamed he would save a former enemy, but seeing the other injured, he still offered help.