He looks like SpongeBob, but is shaped like Plankton. He has Plankton's voice but has SpongeBob's thoughts and personality.
Role in episode
He forms when Plankton inserts his mind connector into SpongeBob's heart. Plankton is being arrogant about his success, leading him to forget about his mission, and thus leading him to turning into Plankton SquarePants.