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