https://www.polygon.com/2018/11/19/18102626/pokemon-lets-go-how-to-style-your-eevee-or-pikachu

One of the most delightful parts of Pokémon: Let’s Go! is bonding with your partner, Pikachu or Eevee. Your new best friend rides around on your shoulder or the top of your hat, toughs out hard battles so you won’t worry about it, learns special techniques so you don’t have to sacrifice a move slot, and gives you adorable high-fives after defeating gyms. The least you can do is spruce it up with some new clothes and hair!

There’s plenty of different outfits for both your trainer and partner Pokémon, as well as unique hairstyles for Pikachu and Eevee’s fur.