Pikachu is the player’s partner in Pokémon Yellow Version: Special Pikachu Edition.
Unlike Pokémon Red and Blue, Pokémon Yellow gives the player Pikachu at the beginning of the adventure. Pikachu follows the protagonist outside its Poké Ball and reacts to the player, reflecting the character’s prominent role in the anime.
The game was released in Japan in 1998 and internationally from 1999 to 2000. It is an enhanced version of the first-generation games, using the anime’s visual influence while retaining the Kanto setting and original Pokémon roster.
Players can also obtain Bulbasaur, Charmander, and Squirtle during the story, but those Pokémon are not the starting partner in Yellow. That difference is why Pikachu is the single canonical answer to this question.