Hinted at in a few interviews with media outlets, a new feature for Pokémon Let’s Go! Pikachu and Let’s Go! Eevee has been fully revealed in a new trailer.
Battles are limited to that single Pokémon that the trainer is a master of and their Pokémon may have moves to counter enemies that need to be planned for. If you earn the title of master for a specific species Nurse Joy may use that title to greet you when you come to a Pokémon Center.