antagonist antagonist /ænˈtæɡənɪst/

noun

1. () Someone who offers opposition  ( opposer , resister , adversary , antagonist , opponent )
   someone who offers opposition
     The antagonist in the story constantly challenged the protagonist's goals.
   The antagonist in the story constantly challenged the protagonist's goals.
   
   
2. A drug that neutralizes or counteracts the effects of another drug  ( antagonist )
   a drug that neutralizes or counteracts the effects of another drug
   
3. A muscle that relaxes while another contracts  ( antagonist )
   a muscle that relaxes while another contracts
    When bending the elbow the triceps are the antagonist
   when bending the elbow the triceps are the antagonist