Sheldon J. Plankton, more commonly known as Plankton, is one of the ten main characters of the SpongeBob SquarePants franchise. He is a planktonic copepod who runs the Chum Bucket restaurant alongside Karen, a waterproof computer who is his sidekick and wife. The Chum Bucket is highly unpopular in Bikini Bottom because it serves chum, a type of bait made from fish meat.

He is the main antagonist of the franchise. There are also several episodes in which Plankton acts as a sympathetic anti-hero. Plankton despises his rival, Mr. Krabs, for running a successful restaurant because his own business rarely gets any customers. His main goal is to steal Krabs' secret formula for Krabby Patties and running Mr. Krabs out of business. Karen gives him a variety of plans to steal it, but their efforts always end up failing in the process.

Plankton is voiced by Mr. Lawrence, one of the show's staff writers. Stephen Hillenburg originally planned for Plankton and Karen to be one-time characters, but Lawrence wrote them into many episodes and proved that they could work as the main villains.[5] Both Plankton and Karen first appear in the episode "Plankton!"

Plankton succeeds at stealing the formula in The SpongeBob SquarePants Movie, but his goal of world domination is eventually thwarted by SpongeBob. In "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 for real, meaning that Plankton had a part in the creation of the Krabby Patty.

More facts here: https://spongebob.fandom.com/wiki/Sheldon_J._Plankton