In the Pokémon TV series, Ash receives Pikachu from Professor Oak as his first partner.
Ash arrives late at Professor Oak’s laboratory on the morning he is meant to choose his first Pokémon. The three traditional Kanto starter Pokémon—Bulbasaur, Charmander, and Squirtle—have already been taken, leaving Pikachu as the available choice. Pikachu is an unusual partner because it initially refuses to stay inside its Poké Ball.
Their relationship develops when Ash protects Pikachu from wild Spearow. The moment establishes the friendship that becomes the central partnership of the anime. Pikachu remains Ash’s most recognizable companion throughout the series.
Pikachu was not Ash’s first capture. Ash receives Pikachu from Professor Oak, whereas Caterpie becomes his first confirmed wild capture. The three Kanto starters are also associated with Ash in the anime, but he does not receive any of them as his initial partner.