Plankton's cellmates are a group of prisoners that only appear in the episode "Jailbreak!." They worship Plankton since his chum is helpful in pulling off crimes. They also help Plankton break out of the Bikini Bottom Jail and steal the Krabby Patty formula.
There are many of Plankton's Cellmates but the main ones (that also stay in the same cell as Plankton) include the Purple Cellmate, who is a large purple fish that has a tattoo of Plankton. He also reveals that he used chum to disguise his face and scare off the cashier out of his money. The Light Blue Cellmate who is a light blue fish that is tall in height. He reveals that he just uses chum as a gas bomb to scare out all of the people in the bank. Reggie is a short green fish with a large mouth. He also helps create the chum by blowing his nose on a napkin. The Whale Cellmate, who is a sperm whale with a scar on his face. He is used as a battering ram to help break into the Krusty Krab. His appearance is similar to Pearl Krabs. Lastly, the Squid Cellmate who is a light blue squid with a beak and a scar on his face.
Plankton's Cellmates are all brought together at first when they decide to follow along Plankton. They all manage to break out of the jail through the front due to all of the policemen getting themselves stuck in a wall. The Cellmates use the whale prisoner to break into the Krusty Krab and it seems that Plankton was finally able to get the Krabby Patty secret formula. However, when Plankton attempts to grab it, it was a trap and revealed to be an entire squad of policemen hiding in the safe. Mr. Krabs tells Plankton that SpongeBob already warned him about Plankton and his Cellmates attempting to break in and hid all of the policemen in the safe. Plankton along with his cellmates are then taken away back to jail.
Dennis also appears as one of Plankton's cellmates.
One of Plankton's jail partners is a whale that looks like Pearl. The only exception is, he has no hair.
When the Purple Cellmate used chum to disguise his face and freak out the cashier, his face greatly resembles Squidward's.