Well, at the moment I don’t think Alola even has a Champion. At the point I’ve reached in the story (obviously I’m ahead of what I’ve actually written, since writing takes time), I’ve learned that Professor Kukui is currently working to form a Pokémon League in Alola; it seems likely that the creation of the League and the selection of Alola’s first Champion are going to be focal points of the rest of the story. That being the case, it seems possible that the story could end with the player being appointed to the position. Kukui himself isn’t out of the question either, since he’s clearly far more martial than other Professor characters. That Hapu chick also seems like a good bet, since she’s clearly very powerful, but doesn’t wear a Captain’s insignia and doesn’t really act like a Kahuna either (she doesn’t hang out on just one island). She just shows up from time to time, helping people and offering advice, which is consistent with the role Champions tend to have in Pokémon games before their formal introduction. If I were a betting man, my money would be on her.