Antagonist

n. adversary, opponent; one who opposes; (Pharmacology, Medicine) drug or substance that acts against or annuls the effect of another substance

In literature, the antagonist is that against which the main character or protagonist contends. An antagonist is often a Villain, but may be a force of nature, set of circumstances, an animal, or other force that is in conflict with the protaganist.
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Antagonist /an·tag·o·nist/ 1. a substance that tends to nullify the action of another, as a drug that binds to a cell receptor without eliciting a [more]Antagonist - Community and Resources

Noun
1. someone who offers opposition
(synonym) adversary, opponent, opposer, resister
(antonym) agonist
(hypernym) person, individual, someone, somebody, mortal, human, soul
(hyponym) dueler, dueller, duelist, duellist
2. a muscle that relaxes while another contracts; "when bending the elbow the triceps are the antagonist"
(hypernym) antagonistic muscle
3. a drug that neutralizes or counteracts the effects of another drug
(antonym) synergist
(hypernym) drug
(hyponym) estrogen antagonist, tamoxifen

n. antagonist, adversary, opponent; one who opposes