From Bulbapedia, the community-driven Pokémon encyclopedia.
Red's Pikachu is the sole starter Pokémon in Pokémon Yellow; it is based on Ash's Pikachu from the Pokémon anime, which Yellow version is loosely based on. This Pikachu first appears along the outskirts of Route 1, where Professor Oak will encounter it and catch it. The player receives this Pikachu after they go into Professor Oak's laboratory and attempt to take the starter Eevee, which Blue will take instead.
Pikachu replaces all of the other usual starter Pokémon, Bulbasaur, Charmander, and Squirtle. However these three other Pokémon can be received during in-game events over the course of the game, which reflects how Ash Ketchum eventually obtained all three starters in the anime. This Pikachu is also the sole Pikachu in all of Pokémon Yellow, and other Pikachu must be traded from other games
After battling against Blue for the first time, the Pikachu will choose to stay out of its Poké Ball and follow the player around everywhere, unless it faints, if it is put into storage, or traded. If the player decides to speak to Pikachu, a small image of Pikachu's current emotion will pop up. The image also shows how much the Pikachu likes the player. This makes Pokémon Yellow the first game to show Pokémon happiness, which would become more prominent in Generation II. Pikachu's happiness doesn't have many uses in the game, however it is vital if the player wishes to receive a Bulbasaur in Cerulean City. Besides showing Pikachu's emotions and happiness, the image can also show several other 'events' such as when Pikachu learns Thunderbolt it will shock the player.
It is a notable fact that this Pikachu will never evolve into a Raichu. If it is offered a Thunderstone it will refuse it. If the player wishes to own a Raichu for either their Pokémon team or completion of the Pokédex, the player will have to trade for a Raichu in another Generation I or Generation II game. If Pikachu is traded to another game, evolved and traded back it will no longer follow the player.
Pikachu is also able to learn Surf if transferred into Pokémon Stadium, and will be able to participate in the Surf Minigame if it does know the move.
In Generation II
In Mt. Silver, Red can be battled. On his team is a level 81 Pikachu that knows Charm, Quick Attack, Thunderbolt and Thunder. As such, Pikachu was the highest leveled Pokémon that could be battled in the game series until three of Pearl's Pokémon surpassed it in Platinum Version. This Pikachu is likely to have been based on Red's Pikachu from Pokémon Yellow.
If Red's Pikachu from Yellow is traded to a Generation II game, it will be holding a Light Ball.
In the manga
The Yellow chapter is based off of Pokémon Yellow. The main character, Yellow, owns a female Pikachu named Chuchu. Red also owns a Pikachu named Pika, however it is unlikely that his Pikachu is based on the Pikachu received in Pokémon Yellow, as the first volume of Pokémon Special was first released on August 8, 1997 in Japan and Pokémon Yellow was first released on September 12, 1998 in Japan.
- Pikachu has several 'in-game events' where it can show an emotion other than how happy it is, they include
- If Pikachu is infected by a status condition, it will appear weak or in pain.
- If Pikachu learns Thunder or Thunderbolt, it may occasionally shock the player if talked to.
- In Pewter City's Pokémon Center, there is a Jigglypuff that will Sing a lullaby if talked to, Pikachu will fall asleep and not move unless awakened by the player. The center's nurse will also comment on Pikachu's sleeping if talked to.
- If the player first goes into Bill's lighthouse and discovers that he has turned into a Pokémon, Pikachu will approach him and look confused. When he reappears in human form, Pikachu will appear to be shocked.
- If the player walks into the Pokémon Fan Club in Vermilion City, Pikachu will approach a Clefairy and fall in love. This may reference their future as members of the Fairy Egg Group, or it could be a reference to the fact that Ash's starter Pokémon was originally going to be Clefairy.
- If the player uses a fishing rod, then checks Pikachu, it will appear to have a bucket over its head.
- In Generation IV, there is a park called Amity Square where players can walk around with a Pokémon following them. However only certain kinds can follow the player, Pikachu is one of the few and may be reference to Red's Pikachu in Pokémon Yellow. However a following Pokémon will not show the player its current emotion when talked to, but may give out something that it found.