Walkthrough: After defeating all Elite Four Members, the center portal will open for your player to access the Champion's Hall. Since you are the first person to take on the Pokemon League, there is no champion and the seat is empty. But if you should sit in said chair, Professor Kukui will appear and inform you that you are not yet the champion of Alola. Instead, you must face on last trainer: himself. And be prepared, as he is one of the strongest trainers in all of the game.
Trainers:
Professor Kukui (Rowlet Starter) - Level 57 Lycanroc, Level 56 Ninetales, Level 56 Braviary, Level 56 Magnezone, Level 56 Snorlax, Level 58 Incineroar w/ Firium Z
Professor Kukui (Litten Starter) - Level 57 Lycanroc, Level 56 Ninetales, Level 56 Braviary, Level 56 Magnezone, Level 56 Snorlax, Level 58 Primarina w/ Waterium Z
Professor Kukui (Popplio Starter) - Level 57 Lycanroc, Level 56 Ninetales, Level 56 Braviary, Level 56 Magnezone, Level 56 Snorlax, Level 58 Decidueye w/ Grassium Z
After defeating Professor Kukui, you will go into the Hall of Fame as the first official Alola Region Champion.