Plankton's cousins - Hillbillies seen in the episode "Plankton's Army." They are cousins of Sheldon J. Plankton, but are not evil geniuses like him. They all seem to like root beer. Some of them are named: Zeke, Rufus, Jeke, Billy Bob, Billy Jim, Billy Billy Bo-Illy Bonana Fanna Fo-Filly, Doug, Enis, and Rainchild.
Clem - Another member of the Plankton family. He is Plankton's illiterate cousin.
Gordon Plankton - Plankton's father and Karen's father-in-law. He appeared in the book Hooray for Dads! He sometimes can embarrass his son, similar to Mr. Krabs embarrassing Pearl.