Pikachu is a yellow mouse-like Pokémon with powerful electrical abilities. In most vocalized appearances, including the anime and certain video games.

Stoty of FUNCAT Pikachu

Ash, a Pokémon trainer travelling from Kanto region till to Alola.
Professor Oak showed Pikachu and said"Here's your starter. You can start adventure with Pikachu". From there on the Pokemon story go long way.