Wiktionary
n. A person in a secondary role, specifically the second most important character (after the (l/en: protagonist)).
Wikipedia
In literature, the deuteragonist (from , deuteragōnistḗs, second actor) is the second most important character, after the protagonist and before the tritagonist. The deuteragonist may switch from being with or against the protagonist depending on the deuteragonist's own conflict/plot.
Usage examples of "deuteragonist".
Billy Barf, whose acquaintance with anything Italian was limited to the deuteragonist of Donkey Kong and a few canned-pasta commercials, insisted on speaking with his imperfect idea of an ethnic accent until Isaiah Two Four, detecting not only its inauthenticity but also its potential for insult, drew the young band eponym aside for a word or two, though Ralph Jr.