In literature, the deuteragonist or secondary main character (from Ancient Greek: δευτεραγωνιστής, deuteragōnistḗs, second actor) is the second most important character, after the protagonist and before the tritagonist. The deuteragonist may switch between supporting and opposing the protagonist, depending on the deuteragonist's own conflict or plot.
Καὶ τό τε τῶν ὑποκριτῶν πλῆθος ἐξ ἑνὸς εἰς δύο πρῶτος Αἰσχύλος ἤγαγε καὶ τὰ τοῦ χοροῦ ἠλάττωσε καὶ τὸν λόγον πρωταγωνιστεῖν παρεσκεύασεν
Kai to te tōn hypokritōn plēthos ex enos eis duo prōtos Aiskhilos ēgage kai ta tou khorou ēlattōse kai ton logon prōtagōnistein pareskeuasen
Thus, it was Aeschylus who first raised the number of the actors from one to two. He also curtailed the chorus and made the dialogue be the leading part.
Aeschylus' efforts brought the dialogue and interaction between characters to the forefront and set the stage for other playwrights of the era, like Sophocles and Euripides, to produce many iconic plays.
Because Ancient Greek drama involved only three actors (the protagonist, deuteragonist, and tritagonist) plus the chorus, each actor often played several parts. For instance, in Sophocles' Oedipus Rex, the protagonist would be Oedipus, who is on stage in most acts, the deuteragonist would be Jocasta (Oedipus' mother and wife), as well as the Shepherd and Messenger. This would be because Jocasta is certainly a major role—acting opposite Oedipus many times and occupying a central part of the story—and because the Shepherd and Messenger are onstage when Jocasta is offstage.
In literature, the deuteragonist often assumes the role of "sidekick" to the protagonist. In Mark Twain's The Adventures of Huckleberry Finn, the protagonist is Huck and the deuteragonist, his constant companion, is Jim. In this story the tritagonist would be Tom Sawyer. Conversely, the deuteragonist could also be a particularly visible antagonist, normally whom the actual antagonist hides behind. In some cases, the deuteragonist is a sidekick who is also used as a foil for the protagonist, in order to more greatly enhance the powers or strengths of the main character. Dr. John Watson, for example, in the Sherlock Holmes series by Sir Arthur Conan Doyle, is an educated and intelligent gentleman of professional standing, and yet his own intelligence is of too rigid a stance to embrace fully the kind of possibilities of which the more maverick Sherlock Holmes is capable.