Sheldon J. Plankton

"Oh man! I hate this guy, Sheldon J. Plankton! This guy must've tried to steal the Krabby Patty formula so many times. Damn it. Well, at least I can squash him, 'cause he's too tiny to succeed."

--Su Ji-Hoon, The Tiniest Foe

Sheldon J. Plankton, more commonly known as just Plankton, is Mr. Krabs' business rival. Plankton despises his rival Eugene Krabs for running a successful restaurant because his restaurant the Chum Bucket is unsuccessful. Plankton runs the restaurant alongside his sapient computer wife Karen, who is usually the source of his evil plans. He is one of the main antagonists of the SpongeBob SquarePants television series. He acts as the main antagonist of a SpongeBob SquarePants animated movie, The SpongeBob SquarePants Movie, the secondary antagonist of a SpongeBob SquarePants stop-animated Christmas special, "It's a SpongeBob Christmas!," and the deuteragonist of another SpongeBob SquarePants animated movie, The SpongeBob Movie: Sponge Out of Water. In some episodes that mainly focus on Plankton, he is portrayed as an anti-hero rather than a villain. He is voiced by Mr. Lawrence and first appeared in the episode "Plankton!"

His primary goal is to steal the Krabby Patty secret formula and put Mr. Krabs out of business. Plankton succeeds at stealing it in the movie, but his goal of world domination is eventually thwarted by SpongeBob. In a SpongeBob SquarePants episode, "Friend or Foe," it is revealed that Mr. Krabs and Plankton were best friends during their childhood, but a dispute over the Krabby Patty secret formula ended their friendship, meaning that Plankton had a part in the creation of the Krabby Patty.

In When the Cold Breeze Blows Away, he is a minion of Lola Loud that uses mind control using buckets to serve for North Korea, until he was then rescued by the PAW Patrol, and soon, he went to work for Lana Loud to use buckets to serve for South Korea instead after such promise to do good things from now on.

He will be voiced by Mr. Lawrence in a Korean drama version of When the Cold Breeze Blows Away.