Very good question. The problem is that most assume that the main character is the "hero" rather than an imperfect individual or even worse, an "anti-hero". I'd say technically that the protagonist of a novel is the main character, even though the idea of an antagonist has negative connotations. The antagonist certainly may "antagonize" the protagonist, for sure!
Here's the def for Protagonist:
the leading character or one of the major characters in a drama, movie, novel, or other fictional text.
the main figure or one of the most prominent figures in a real situation.
"in this colonial struggle, the main protagonists were Great Britain and France"
synonyms: chief/central/principal/main/... character, chief/central/principal/main/leading participant, chief/central/principal/main/leading figure, chief/central/principal/main/leading player, principal, hero/heroine, leading man/lady, title role, lead
"the protagonist in the plot"
an advocate or champion of a particular cause or idea.
"a strenuous protagonist of the new agricultural policy"
synonyms: champion, advocate, upholder, supporter, backer, promoter, proponent, exponent, campaigner, fighter, crusader;
Here's the def for Antagonist:
noun: antagonist; plural noun: antagonists
a person who actively opposes or is hostile to someone or something; an adversary.
"he turned to confront his antagonist"
synonyms: adversary, opponent, enemy, foe, rival, competitor; More
a substance that interferes with or inhibits the physiological action of another.
a muscle whose action counteracts that of another specified muscle.
NOTE: THE ANTAGONIST IS AN "ADVERSARY".