Reggie is an odd, light green fish (possibly an eel or a barracuda) that has a large mouth. He also has a light orange fin and is always seen wearing jail clothes.
Reggie is first seen when he takes a milk carton from Plankton. After finding out who Plankton really was, Reggie is more than grateful to assist his hero in stealing the Krabby Patty Secret Formula. Reggie also provides an ingredient to help create chum which aided in everyone's escape. Reggie the helps break into the Krusty Krab but he is eventually arrested along with the others.
Reggie is the only one of Plankton's cellmates to have his real name revealed in the episode itself.