Antagonist

Antagonist-n
1-somebody or something opposing or in conflict with another
2-a major character in a book, play, or movie whose values or behavior are in conflict with those of the protagonist or hero
3-a substance, often a drug, that nullifies the effect another substance has on the body
4-a muscle that acts with and limits the action of another muscle

Synonyms
rival
adversary
opponent
enemy
contender
competitor
friend (antonym)