Pikachu is the mascot of the Pokémon franchise and one of the protagonists of the Pokémon anime. He is a small, stubby Electric-type mouse Pokémon.
In SpongeBob UnCanon, Ash's Pikachu is one of the playable protagonists alongside SpongeBob and Patrick, as well as the fifteenth main boss of the game. He first appears in the Hoenn Region level where he meets the two best friends, but battles them, assuming they are unidentified Pokémon. Later, Pikachu decides to help SpongeBob and Patrick chase Plankton to get the Krabby Patty formula back. Throughout the game, he helps them battle Plankton's evil Team Copepod forces in the remaining worlds they travel to until they head back to Bikini Bottom.
In the final level of the game, known as "Dirty Bottom", Pikachu, Patrick and SpongeBob are tested for their strength as Plankton sends out waves of Team Copepod soldiers for them to battle. Pikachu, however, caught Plankton's attention by his tenacity and determination, thus amusing him. After the trio finished off the fifth wave, Plankton took it upon himself to have Pikachu as his new member. He shoots a metallic pair of zooplankton antennae, which happens to be a mind control set, onto Pikachu's head and proceeds to hypnotize him. SpongeBob and Patrick try to help Pikachu break free, but fail. Pikachu's eyes begin glowing red as he turns against the two and becomes more aggressive towards them. In the Chum Bucket Prison area of Dirty Bottom, he is a boss. He is seen jumping in front of SpongeBob just as he was about to leave the surveillance room with the key card for Patrick's cell, which he was going to use to free Patrick. Plankton, angered by SpongeBob's easy escape, commands Pikachu to kill him to make sure he doesn't foil his plan. SpongeBob fights Pikachu until the mind control antennae are destroyed, thus snapping Pikachu out of Plankton's corrupting influence. He turns back to SpongeBob's side and then helps him and Patrick defeat Plankton and Team Copepod. After they foil Plankton's plot and recover the secret formula, he goes back to his world with Ash Ketchum and the rest of the Pokémon crew as he bids farewell to SpongeBob and Patrick.
Before the end credits, SpongeBob receives a package in the mail from Professor Oak. He opens it to reveal a Pokeball that summons a Pikachu of his own. The new Pikachu becomes SpongeBob's new friend. This sparks the beginning of the television series spin-off.