Deuteragonist. The second guy. No, not the co-protagonist, who shares equal billing with their other co-protagonist. Not the Supporting Protagonist, who is a main character but not the focus of the story. Not the Decoy Protagonist either. This is the second person the show revolves around, a character whose actions drive the plot just as much as those of the protagonist.
The deuteragonist (from Greek: second actor) is the second most important character in the story; the first is, of course, The Protagonist. This person can be either with, or against the protagonist—thus sometimes pulling double duty as a major antagonist or rival to the protagonist; though they are rarely the main "villain" in this sort of scenario, they may be a high ranking minion. If on the same side as the protagonist, they can be a sidekick, lancer, or love interest as long as they are given enough screen time independent of the main protagonist. In case of a Geodesic Cast, the Deuteragonist will be the one who corresponds to the first protagonist.