There are many of Plankton's cellmates, but the main ones include the purple cellmate, who is a large purple fish who 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 is a light blue fish who is quite tall. 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 moray eel with a large mouth which can bite really strongly. He also helps create the chum by blowing his nose on a napkin.
Lastly, the squid cellmate is a light blue squid with a beak and a scar on his face.
Role in episode
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 is 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 hides 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 who looks like Pearl. The only exception is that he has no hair.
When the purple cellmate uses chum to disguise his face and freak out the cashier, his face greatly resembles Squidward's.