Pikachu became the Pokémon mascot and Ash Ketchum's main partner in the original anime.
The anime's production team selected Pikachu because its design was cute, approachable, and appealing to both boys and girls. Choosing Pikachu also avoided favoring one of the three game starters, since different players could choose Bulbasaur, Charmander, or Squirtle in the original games.
In the first episode, Pikachu initially refuses to obey Ash and will not remain inside its Poké Ball. Their relationship changes after Ash protects Pikachu from a flock of Spearow. That story established the friendship that drove the anime's early seasons and made Pikachu recognizable even to people who had never played the games.
Pikachu is an Electric-type Pokémon whose name combines Japanese sound-symbolic words associated with sparkling electricity and a mouse's squeak. Pichu, its pre-evolution, was added later in Pokémon Gold and Silver.