Little is known of Plankton's conception. His parents have not been seen in the show at all. However, it is known that he is the same age as Eugene Krabs because they grew up together and were close friends in their youth. Krabs and Plankton split over a disagreement regarding the Krabby Patty recipe and have remained bitter rivals since. At some point in his life, Plankton opened the Chum Bucket. How the restaurant has remained open for a long duration of time is unknown as it is almost always empty. Partially as a result of his floundering business, Plankton constantly plots to steal the Krabby Patty formula in order to destroy Krabs, develop a profitable business, and conquer the world.
In Plankton's Army, a host of Plankton's Cousins were brought together by Plankton in an attempt to steal the formula. Unlike Plankton, these cousins were not intelligent and were portrayed as stereotypical Southern hillbillies. Other than this, little about Plankton's family is known.
Plankton is actually the only main character in the series to be married, albeit to a computer. Plankton apparently built Karen himself, developing her overtime. Karen has sometimes helped Plankton in his schemes, especially in earlier episodes. Since The SpongeBob SquarePants Movie, however, her role has been less supportive of his constant obsession with the formula. For example, She pushed Plankton to redecorate the worn-down Chum Bucket in Bucket Sweet Bucket.
Some episodes explore the duo's relationship in greater detail. In Single Cell Anniversary, Karen kicks Plankton out of the Chum Bucket for forgetting their anniversary. So, he recruits SpongeBob to help him make up for it and resolves the conflict with a heartwarming serenade that wins back Karen's heart. Plankton replaces Karen with three combined gizmos in Komputer Overload in another attempt to steal the formula. However, his devices malfunction within feet of he barging straight into the Krusty Krab. A defeated Plankton begs Karen to forgive him and after mulling it over, she does. In Karen 2.0, he fully replaces the original Karen with a new model, going as far as putting the old Karen in a box on the side of the street. SpongeBob and Mr. Krabs find her and employ her at the Krusty Krab. Eventually, the original Karen attacks the replacement and beats her down after the replacement hurts Plankton. Again, Plankton and Karen end with a happy reunion.
Plankton is designed somewhat like actual plankton. He has two antennae, one eye, and a mouth. He is very small in size and is easily stepped on. Plankton rarely wears clothing, probably due to his size.