The Plankton Family - A large number of plankton are 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 Willy Banana Fana Fo Filly, Doug, Enis, and Rainchild.
Clem - Another member of the Plankton family. He is Plankton's illiterate cousin.
Mr. Plankton - Plankton's father. He appeared in the book Hooray for Dads. He sometimes can embarrass his son, similar to Mr. Krabs' embarrassing Pearl.
Mr. Puff - Mrs. Puff's deceased husband; he was turned into a lamp by a fisherman seen in the episode "Krusty Love." In that episode, "Mr. Krabs asked what happened to Mr. Puff and SpongeBob said that Mrs. Puff does not like to talk about her dead husband. In The SpongeBob SquarePants Movie, it hints that Mrs. Puff's husband was killed and made into a lamp.
Pa Cheeks - Sandy's father, who was referenced in "Sandy, SpongeBob, and the Worm." Sandy calls him "Pa" or "Pappy." He made a live appearance in the book "Hooray for Dads." He looks a lot like Sandy except with a gray mustache and some other male attributes.
Sandy's Scottish Cousin - This family member appeared in the episode "Stanley S. SquarePants" as one of the family members of the characters who was turned down by Mr. Krabs for a job at the Krusty Krab. His fur was more unmaned and he had a pipe, a bagpipe, and a kilt. He resembles a Scottish squirrel, which could indicate that Sandy's ancestors lived in Scotland before Texas.
Rosie Cheeks - Sandy's great aunt, Mr. Cheeks' aunt. Rosie discovered oil in Spindletop, Texas.