From Bulbapedia, the community-driven Pokémon encyclopedia.

Red's Pikachu is the only starter Pokémon of Kanto in Pokémon Yellow, a 'sister game' of the original Pokémon Red, Green, and Blue that is based on the Pokémon anime. In the anime the main character Ash Ketchum's starter Pokémon is a Pikachu.

When the player tries to go into Route 1, the player will be stopped by Professor Oak who encounters a wild Pikachu. After capturing the Pikachu, Professor Oak will direct Red to his lab in Pallet Town, where the rival (Blue) awaits.

Since the ordinary starter Pokémon have been taken, the player will receive the Pikachu, while the rival takes the Eevee. After the first battle with Blue's Eevee, the Pikachu will pop out from its Poké Ball and follow Red about on foot. In the beginning, it will be angry, although its happiness can be improved when it gains a level or when an item, such as Potion, is used on it. However, its happiness can also drop, such as when it faints.

The player can choose to turn to his Pikachu and talk to it, resulting in a small image of the Pikachu's current emotion to pop up. The image will change depending on its happiness or if a small event happens like if its falls asleep from a Jigglypuff's song in Pewter City's Pokémon Center. It's also notable that it will shock the player when it learns Thunderbolt or Thunder, and falls in love with the Clefairy in Viridian City's Pokémon Fan Club. Another speical scene happens after the player goes fishing and then talks to pikachu before moveing; a scene showing Pikachu lifting a overturned bucket of of itself.

In Gold, Silver, and Crystal, Red can be battled on the final floor of Mt. Silver and has Pikachu on his team. Here, its level is 81 and its moveset consists of Charm, Quick Attack, Thunderbolt and Thunder.

It is unique due to the fact that it is the only Pikachu found in Pokémon Yellow, as well as being the only starter Pokémon. In addition, when offered the Thunderstone, Red's Pikachu will refuse to evolve into Raichu just like Ash's Pikachu in the anime. The Pikachu can also learn Surf, and can be traded between Generation I and Generation II, holding a Light Ball. Moreover, it can also be transferred over to Pokémon Stadium 2 to be played in the game Pichu's Power Plant.

Various Pikachu in the manga have been based off it, including both Red's and Yellow's Pikachu in Special and Isamu Akai's Pikachu in the Pocket Monsters manga.