Pikachu replaces all of the other usual starter Pokémon, Bulbasaur, Charmander, and Squirtle, but these three Pokémon can be received during events later in the game (which reflects Ash Ketchum's eventually obtaining all three of Kanto's starters in the anime.) This Pikachu is also the only Pikachu found in Yellow; other Pikachu must be traded in from compatible 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 unless he faints, is put into storage, or is traded. Should the player speak to Pikachu, a small animation of his current emotion will pop up; this image can also be an indication of how much the Pikachu likes the player. (This makes Pokémon Yellow the first game to show Pokémon friendship, a mechanic that would become more prominent in Generation II.) Although the player's friendship with Pikachu does not have many uses in the game, it is vital if the player wishes to receive a Bulbasaur in Cerulean City. Besides showing Pikachu's emotions and friendliness, the game also shows animations of several other events, such as Pikachu learning Thunderbolt. These also include:
If talked to right after the player loses to Blue in the Pokémon Lab, Pikachu will turn its back to the player.
If talked to right after the player wins against Blue in the Pokémon Lab, it will appear uninterested in the player.
If Pikachu is affected by a status condition, it will appear weak or in pain. If it is asleep, it will appear asleep when talked to.
If Pikachu has just learned Thunder or Thunderbolt, it may shock the player if talked to.
In Pewter City's Pokémon Center, there is a Jigglypuff that will Sing a lullaby if talked to. After listening to the Jigglypuff, 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, instead of healing the party. The cable club will also be inaccessible. Until the player awakens Pikachu, it cannot be deposited into the PC, have items used on it (excluding the Poké Flute), or moved in the party.
When the player goes to Bill's house and discovers that he has turned into a Pokémon, Pikachu will approach him and look confused. When Bill reappears in human form, Pikachu will appear to be shocked.
If talked to right after the player catches a Pokémon, he will make a V with his fingers to represent victory, similarly to how Ash's Pikachu posed after Ash caught Caterpie.
If talked to after the player loses a battle, Pikachu will appear disappointed in and unsure of its Trainer.
Notably, reflecting the actions of Ash's Pikachu, this Pikachu will refuse a Thunderstone given to it in Yellow, and, if talked to after the player tries to use the stone, will shake its head in refusal. It will not evolve unless traded to another cartridge. (The player can obtain Raichu in Yellow either by trading one in or by using a Thunderstone on a Pikachu traded from another game.)
After the player stores Pikachu in Bill's computer, Pikachu complains and becomes less friendly toward the player. The player is also unable to release Pikachu; if release is attempted, Pikachu will complain.
In battle, Pikachu's animations and cry are different from other Pikachu, its voice based on Ikue Ohtani's. In the Virtual Console release of Pokémon Yellow, Pikachu can also participate in the Surf Minigame.
Red can be battled in Mt. Silver, with a level 81 Pikachu on his team. At the time, his Pikachu was the highest leveled Pokémon that could be battled in the main series. This Pikachu is likely to have been based on Red's Pikachu from Pokémon Yellow.
Although Red doesn't use his Pikachu when battled in Pokémon Stadium 2, he still makes a cameo appearance during the game's end credits, battling Blue's Eevee.
When brought into Pokémon Stadium 2, the player's Pikachu from Pokémon Yellow still uses special Pikachu voice effects rather than its cry.
In Generation IV
Red's challenge at Mt. Silver returns in Pokémon HeartGold and SoulSilver. He still uses Pikachu in these games; he is now level 88. Again, at the time, Pikachu was the highest level Trainer-owned Pokémon in the series.
Pikachu Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.
When battling against Red, Pikachu appears to come from inside his Poké Ball, contradicting his dislike of doing so in Pokémon Yellow.
Although Pikachu initially resides inside of its Poké Ball until the player has battled with the rival for the first time in Pokémon Yellow, it still enters the first battle from outside of the Ball.
Red's Pikachu is the first starter Pokémon in the main series that will not evolve unless it is sent to another game.
Other Pikachu when transferred from other Generation I titles to Yellow will behave like normal Pokémon, as opposed to following and interacting with the player. This is because the game checks for a Pikachu in the player's party with a trainer ID and OT that matches the player. If a second Pikachu were obtained through illegitimate means to have these matching variables, the game would use whichever Pikachu appears first in the player's party.
For more information on this Pokémon's species, see Pikachu.