The poet Thespis revolutionized the early stages of ancient Greek theatre by being the first to introduce one actor standing apart from the chorus, with deeper dialogue and greater development. Then Aeschylus came around and made it two actors, and Sophocles popped in to make it three. The initial and most important of these characters was the protagonist, then the deuteragonist for the secondary character and Sophocles' third character was called the tritagonist. Any or none of these characters could also be the antagonist, which obviously is the bad guy. Now, in those old Athenian times, they would have actors compete to win sort of their equivalent of the Best Actor Tony Award, so the word agonist, meaning "actor", originally could be defined better as "competitor" or "combatant" because they were competing for the theatre laurels. Pro- means that person is the first actor, deu- that they're the second, trit- that they're third, and anti- that they're "against" the other competitors. In Modern English, only protagonist and antagonist remain in use, and their meanings have clearly changed a bit since then.
Adam Aleksic, a freshman studying linguistics at Harvard University, has been described as the internet's sixth most famous etymologist. He has disturbing interests in words, vexillology, geography, board games, limericks, and law, and he loves writing about himself in the third person.
The Etymology Nerd