Deuteragonist

The deuteragonist is "the second most important character of a narrative, after the protagonist and before the tritagonist." The word is the Ancient Greek deuteragōnistḗs, "second actor," a role Aeschylus introduced to Greek drama according to Aristotle's Poetics. The article also calls the role the secondary main character, and notes that it "may switch between supporting and opposing the protagonist": constant companion, active aide or near-equal rival. In Oedipus Rex "the deuteragonist would be Jocasta," with Oedipus as protagonist and the tritagonist playing the Shepherd and Messenger.

For novelists the deuteragonist is the book's second contract: the character whose arc and interiority get weight second only to the protagonist's, and the natural home for the story's other argument. In serial fiction the role is ensemble duty, the second viewpoint that varies chapters and covers ground the lead cannot see, which is the casting decision managing an ensemble cast covers.