Plankton's cellmates are a group of prisoners that only appear in the episode "Jailbreak!" They worship Plankton because 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 stayed in the same cell as Plankton) include:
Purple Cellmate: A large purple fish that has a tattoo of Plankton. He also revealed that he used Chum to disguise his face and scare off the cashier out of his money.
Light Blue Cellmate: A light blue fish that is pretty tall in height. He revealed that he just uses Chum as a gas bomb to scare out all of the people in the bank.
Reggie: A short green fish with a large mouth. He also helped created the Chum by blowing his nose on a Napkin.
Whale Cellmate: A sperm whale with a scar on his face. He was used as a battering ram to help break into the Krusty Krab. His appearance is similar to Pearl Krabs.
Squid Cellmate: A light blue squid with a beak and a scar on his face.
Plankton's Cellmates were all brought together at first when they decide to follow along Plankton. They all managed to break out of jail through the front due to all of the policemen getting themselves stuck in a wall. The Cellmates used the whale prisoner to break into the Krusty Krab and it seemed that Plankton was finally able to get the Krabby Patty secret formula. However, when Plankton attempted to get it, it was a trap and revealed to be an entire squad of policemen hiding in the safe. Mr. Krabs told Plankton that SpongeBob already warned him about Plankton and his Cellmates attempting break in and hid all of the policemen in the safe. Plankton along with his cellmates were 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.