From Longman Dictionary of Contemporary Englishantagonistan‧tag‧o‧nist /ænˈtæɡənɪst/ noun [countable] AGAINST/OPPOSEyour opponent in a competition, battle, quarrel etc → protagonist
Examples from the Corpus
antagonist• Yet, on the whole, the rich man remains the natural antagonist of the poor.• In the fluctuating combination and recombination of groups, old friends and old antagonists come across one another in shifting social contexts.• In recent years the relationship has become complex, but in the early years of each they were antagonists. 