Pokémon Yellow introduced Pikachu following the player as its central version-specific mechanic.
Released in Japan in 1998, Pokémon Yellow was based on Pokémon Red and Green while taking inspiration from the Pokémon anime. Instead of letting the player choose Bulbasaur, Charmander, or Squirtle at the beginning, the game gave the player a Pikachu that followed them outside its Poké Ball.
Pikachu’s mood could change depending on how the player treated it, and the game added events involving Jessie, James, and Meowth from Team Rocket. The player could later obtain the three original starter Pokémon through in-game characters, making Yellow different from the earlier paired versions.
Pikachu-following features later appeared in other Pokémon games, so it is easy to associate the mechanic with newer titles. However, Pokémon Yellow is the classic game most directly identified with making Pikachu a visible companion rather than simply another party member.