The character who is an enemy of the main character?

In traditional literary theory the character who carries the main intention of the narrative (nearly always 'the main character') is called the PROTAGONIST.

The character who contrasts with the PROTAGONIST (who works against the main character) is often called the ANTAGONIST, though some critics prefer to call this character the DEUTERAGONIST.

The big problem with all of this terminology is that it assumes that every literary work can be usefully critiqued with ideas based in the conventions of classical Greek tragedy.

In practice, not all important works have a Protagonist (Shakespeare's Twelfth Night probably does not), and even fewer have a meaningful Antagonist (Macbeth has no single character who can be assigned this role).