Ash received Pikachu from Professor Oak in the first Pokémon anime episode.
In “Pokémon, I Choose You!”, Ash arrives at Professor Oak’s laboratory after the other three starter Pokémon have already been chosen. Oak gives him Pikachu, an Electric-type Pokémon that initially resists entering its Poké Ball and following Ash’s commands.
Their relationship changes when Ash puts himself in danger to protect Pikachu from Spearow. Pikachu then uses a powerful electrical attack to drive away the flock, beginning the friendship at the center of the original anime.
The choice of Pikachu also avoided favoring Bulbasaur, Charmander, or Squirtle, the three selectable starters in the original Game Boy games. Pikachu therefore became the anime’s central partner and the franchise’s mascot.