Pikachu was chosen as the player's partner in Pokémon Yellow.
Unlike Pokémon Red, Green, and Blue, Pokémon Yellow begins with the player receiving Pikachu rather than selecting Bulbasaur, Charmander, or Squirtle. Pikachu follows the player outside its Poké Ball and refuses to evolve with a Thunder Stone during the story.
The design was inspired by the Pokémon anime, where Ash Ketchum travels with a Pikachu that also remains outside its Poké Ball. Yellow therefore acts as an enhanced version of the first-generation games while adding anime-related character appearances and story details.
Pikachu's importance grew beyond its original role as one species among 151. Its distinctive silhouette, expressive face, and anime prominence made it the franchise mascot. Players can still obtain the other original starter Pokémon during the adventure, but Pikachu is the defining partner of Yellow.