After Professor Oak's intervention, he'll lead you to his lab. Your rival, Blue, will already be there, eager to get his first Pokémon. Professor Oak initially intends to give you an Eevee, but Blue, being impatient, takes it for himself. This leaves you with the only remaining Pokémon: a feisty Pikachu!
Pikachu will be your constant companion, following you on the overworld map. Unlike other starter Pokémon, Pikachu cannot evolve in Pokémon Yellow unless you trade it to another game and expose it to a Thunder Stone. However, it's a powerful Electric-type Pokémon and will be invaluable throughout your journey. talk to Pikachu frequently to check its happiness level; a happy Pikachu performs better in battle!