antagonist
     n 1: someone who offers opposition [syn: {adversary}, {opponent},
           {opposer}, {resister}] [ant: {agonist}]
     2: a muscle that relaxes while another contracts; "when bending
        the elbow the triceps are the antagonist"
     3: a drug that neutralizes or counteracts the effects of
        another drug [ant: {synergist}]
Source: WordNet® 2.0