As with previous games, you can re-enter the Pokémon League and fight the Elite Four as many times as you want. Which is a great way to quickly amass experience points and money. When re-matching the Elite Four, their Pokémon are all Level 66 instead of 56-57, but you should be able to handle them.
Afterwards, what usually happens is that you battle the previous Champion again, but you are the one and only Champion, so that doesn’t quite work. Instead, when you reach the Champion’s seat, a randomly selected Pokémon trainer will arrive to try and claim the title from you–hence, the “Champion Title Defense”.
The challengers that can appear include various powerful trainers from the story–and a few odd choices. The first time you re-enter the Pokémon League, you will always face Hau. However, on subsequent visits, the challenger will be random. You can even save next to the seat and reload the game to change the outcome.
By this stage, with all your knowledge from the story and the postgame, you should be perfectly capable of defending your title from all of the possible challengers. However, for your information, here’s a list of the challengers and what Pokémon they use, so you can plan what to do in advance.
After defending your title, you’ll be taken straight outside the entrance of the Pokémon League, skipping the ending credits. If you’ve defended your title at least twice, a pair of girls will be waiting just near the entrance. Speak to them to receive two Sweet Hearts . As in the consumable item, not anything unseemly.
Furthermore, if you’re patient enough to encounter and defeat Gladion at the end of the Pokémon League, something cool will happen if you visit Aether Paradise. As you head towards the northern exit of the indoors entrance area, you’ll find a surprising visitor and a touching conversation will occur.