Sheldon J. Plankton

Sheldon J. Plankton is one of the ten main characters of "SpongeBob SquarePants". He is a planktonic copepod who runs the Chum Bucket restaurant alongside with Karen, a waterproof robot who is Plankton's sidekick and wife.
Plankton debuted in the episode "Plankton!", which aired on July 31, 1999.