There are several synonyms for the word "enemy" which can be used in different contexts. Some common synonyms are foe, adversary, rival, antagonist, and nemesis.
Foe is an old-fashioned term and refers to a person who is openly hostile or has a deep-seated hatred for someone. Adversary is a more formal term used to describe someone who opposes another person or group. Rival refers to someone who is competing against another in a similar line of work or activity.
Antagonist is often used in reference to a character or force in literature or drama that opposes the protagonist. Nemesis refers to a person or thing that inflicts downfall or retribution on another.
Each of these synonyms can be used to convey a nuanced meaning depending on the context in which they are used.