Ash receives Pikachu at the start of the original Pokémon anime.
In the first episode, “Pokémon, I Choose You!”, Ash arrives late to Professor Oak’s laboratory and finds that the usual starter Pokémon have already been chosen. His remaining choice is Pikachu, an Electric-type Pokémon that initially refuses to stay inside its Poké Ball and does not immediately trust him.
Their relationship changes during the episode when Ash risks himself to protect Pikachu from a flock of Spearow. Pikachu then uses an enormous electric attack to drive the birds away. This event establishes the partnership that becomes the emotional center of the original series.
In the video games, players normally choose Bulbasaur, Charmander, or Squirtle at the beginning of Pokémon Red, Green, and Blue. Ash’s Pikachu is therefore an anime-specific starting partner, not one of the three standard game starters. Pikachu’s role helped make it the franchise’s best-known mascot.