Narratively, I don’t think Hau, your game-long rival, showing up at the end of Ultra Sun and Ultra Moon is as compelling as Professor Kukui fighting you as the Alolan Champion. But I do have to give the kid props for being more adaptable than the professor when one of their Pokémon is modular depending on the player’s choice.
Like Kukui, Hau picks one of the Alolan starters depending on which one you choose at the beginning of the game. On his journey he also gets an Eevee, which any Pokémon fan knows can evolve into one of several forms with different types. Depending on which Pokémon he chooses, he evolves his Eevee to accommodate a gap in his team’s typing. So since I chose Popplio, he evolved his Eevee into Leafeon, which gave him a grass-type counter to my water-type starter. Beyond that, his other Pokémon are pretty formidable and each have a strong spread of attacks that can exploit several weaknesses, so there really aren’t a lot of one-trick ponies on his belt for the championship fight.
I do gotta ask, though, why does his starter only have three attacks? Nothing is stopping you from teaching them a fourth move, king. This is how the game works.