Which Pokémon game first let Pikachu follow the player outside its Poké Ball?

Pokémon Yellow was the first Pokémon game to let Pikachu follow the player outside its Poké Ball.

Released in Japan in 1998, Pokémon Yellow is an enhanced version of the first-generation Game Boy games. It was designed partly to reflect the television anime, especially the relationship between Ash and his Pikachu.

Instead of choosing Bulbasaur, Charmander, or Squirtle at the start, the player receives Pikachu. That Pikachu refuses to remain inside its Poké Ball and follows the player on the overworld. Its reactions can be checked by speaking to it, giving the game a relationship feature unusual for the earlier versions.

Pokémon Yellow also changed several story encounters and made Team Rocket’s Jessie and James more prominent. Later games reused the idea of a Pokémon accompanying the player, but Yellow was the first main-series title to establish this recognizable feature.