Pikachu is Ash Ketchum’s first partner Pokémon in the Pokémon anime. Ash receives Pikachu from Professor Oak in the first episode after arriving late to choose a starter Pokémon.
Unlike the three standard first-generation starters, Pikachu initially refuses to cooperate. It stays outside its Poké Ball and resists Ash’s commands, creating the central relationship conflict of the opening episode.
Their bond changes when Pikachu protects Ash from a flock of Spearow. Ash’s willingness to defend Pikachu, followed by Pikachu’s powerful electrical attack, establishes their friendship and sets the tone for the series.
Pikachu is not Ash’s first Pokémon in every sense: the anime later shows him obtaining other Pokémon, and Ash’s early journey includes several captures. The question refers specifically to his first assigned partner, which is Pikachu rather than Bulbasaur, Charmander, or Squirtle.