Pikachu became the mascot of the animated series after being chosen as Ash Ketchum's partner.
Pikachu is an Electric-type Pokémon first introduced in the original Game Boy games. The anime's producers selected it because its design was appealing to both boys and girls and because using one of the three game starters could have favored players who chose a different starter.
In the first episode, Ash receives Pikachu from Professor Oak. Pikachu initially refuses to cooperate, but the pair develop a lasting friendship after Ash risks himself to protect Pikachu from attacking Spearow.
Clefairy was considered for a prominent early role, and Bulbasaur, Charmander, and Squirtle are the original game starters. None, however, became the anime's central mascot. Pikachu's yellow design, expressive face, and recognizable voice helped make it the global symbol of Pokémon.