God fucking dammit.
In vidya, you are always the protagonist. Protagonist = prōtos (first/most important) + agōnistēs (actor(according to the root of the word combatant)). And, when Greeks said actor, they didnt refer to no bloody Tom Cruise or film diva, they referred to the guy who acts. Ie the guy who is the source of the action. Thus, the protagonist is always the main source of action, the one from whose perspective you see the world, while antagonist is the main source of counteraction. Plain and simple. In Wizardry 4, you (Werdna) is the protagonist, and the antagonists are the the heroes from previous games (and yes, protagonists from previous games.) The main source of action is naturally the character from whose perspective we see the story develop, the main character - the protagonist. This goes double for vidya, where action is often very literal. Thus, your character is always the protagonist - you are always the protagonist.
What you folks are talking about is a central figure/key character. Yes, Revan is the most important character in the fictive world of Kotor 2, but for you, the player, the Exile is the most important character - he's the one from whos perspective you see the story, hes the main source of action, and its his god damn story. To use Russian formalist terms, Revan would be in the center of the games fabula (the story as it happened) but Exile is the center of the games syuzhet (story as its told). Thus, Jedi Exile is the protagonist of Kotor 2.
There could be some extreme cases where you arent playing the protagonist (or there are multiple protagonists/protagonist+deuteragonist+tritagonist), but for a huge majority of shit, it works as i explained.