Master Trainers are a unique new feature to Pokémon: Let's Go, Pikachu! & Let's Go, Eevee!. These trainers are the strongest trainers in Kanto and specialise in 1 Pokémon, challenging you to a mirror match. You can only use 1 Pokémon, the same species as them. They are of high level and have various unique and powerful moves which are often super effective against themselves. You cannot use Items and you do not gain Experience from the battle

These Master Trainers require you to think fully about the strategy. However, even though Items cannot be used in battle, you can Mega Evolve your Mega Evolution capable Pokémon and if you have Alolan Forms of various Pokémon, you can even use them against the Master Trainer, which can potentially give you an advantage.

There are also several Master Trainers which do not wish to battle you but just wish to see a strong version of that Pokémon. This requires the Pokémon to have a specific amount of CP. Talk to them and they'll give you the title.

If you manage to defeat them, you will gain their title of Master which can be used when greeted at the Pokémon Center or when you battle with friends locally or online. This can be changed by speaking to the lady in the Game Freak offices in Celadon City

When you have obtained all the Master Trainer Titles, speak to the lady again and you'll receive the title of "Grand Master".