Sheldon J. Plankton or just commonly known as Plankton, is one of the main characters as well as being the arch-enemy and ex-friend to Eugene H. Krabs, and the main antagonist of the series. He is known to usually wanting to steal the Krabby Patty Secret Formula.
Plankton is shown to be very evil mainly in the early episodes. He was the last main character to have a first appearance. He has only gotten the formula just a few times. Especially in the movie where he took over all of Bikini Bottom. His restaurant, the Chum Bucket rarely gets customers. He is the smallest character in the series along with Prawn. Plankton is married to a female computer named Karen Plankton. He also has a large family which are mainly just cousins. He is probably the most evil character in the show, besides Mermaid Man and Barnacle Boy's enemies.