Sheldon Plankton is a major antagonist in Spongebob Squarepants and a member of the Organization.
Plankton is Eugene Krabs's arch rival and the main antagonist of the series, though the two were once best friends, as revealed in "Krabs vs. Plankton" and "Friend or Foe?". He operates a struggling restaurant called the Chum Bucket across the street from the Krusty Krab. His primary goal in the series is to steal the Krabby Patty recipe and drive Mr. Krabs out of business. SpongeBob has also had a few temporary friendships with Plankton, such as in "F.U.N.", when Plankton learns what fun is. Plankton has also been revealed to an immense fear of whales.
Plankton has a computer wife named Karen, who is actually a "W.I.F.E." (Wired Integrated Female Electroencephalograph) Karen apparently has a mind of her own, to the point of making fun of her husband. She usually gives him ideas which he often censors out and takes them as his.