https://www.polygon.com/2018/10/18/17994956/pokemon-lets-go-post-game-master-trainers

As director Junichi Masuda teased for us last week, Pokémon: Let’s Go! will introduce a unique challenge during its postgame. Scattered throughout the world of Kanto are expert trainers, each one the master of a specific Pokémon. Your task? Challenge them all — all 151 of them.

They’re called Master Trainers, and they’ll only battle you once you’ve bested the Elite Four and taken down the Pokémon League. With your Champion belt in hand, you can go through the Kanto region once again to find these characters. They’re designated by a thought bubble above their heads showcasing their favorite Pokémon. Their favorite Pokémon also happens to be the only one on their team, as well as the only one they’re willing to square off against.