Sheldon J. Plankton

Sheldon J. Plankton (simply known as Plankton) is a one-eyed copepod who is married to Karen Plankton. He runs the Chum Bucket restaurant, where he also resides. His restaurant was a failure because it mostly sells chum. He always plans to steal the Krabby Patty secret formula from the Krusty Krab, a nearby restaurant owned by Plankton's arch-rival Eugene H. Krabs.

History
Plankton and Mr. Krabs were great friends when they were young. They created the Krabby Patty together after the most popular burger joint got shut-down. After everyone had a patty it made their stomachs ache and cramp. After they made enough money they got in a fight and each got half of the recipe. Mr. Krabs got the half that had everything on it but the ingredient that made everyones stomachs ache. That ingredient was chum. So ever since then Plankton has been trying to steal the recipe.