As you continue to explore the Kanto region in Pokémon Let’s Go, Pikachu! and Pokémon Let’s Go, Eevee!, you will soon have a Bottle Cap stuffed into your Bag. These Bottle Caps are said to make some people happy when they receive one, but, in reality, they are used for Hyper Training.
This was first introduced in Pokémon Sun and Pokémon Moon, and, after you have beaten the Elite Four and become Champion, you can talk to Mr. Hyper who is stood in the Day-Care Center on Route 5.
If you have a Level 100 Pokémon, you can use a Bottle Cap to boost one of that pocket monster’s IVs (Individual Values) to the maximum value. Or, if you have a Gold Bottle Cap, it will raise all IVs to their maximum value.
It’s worth mentioning that Bottle Caps are rare items, so make sure that you use them on your favourite Pokémon – whether they be normal, Shiny or Legendary Pokémon.