He resembles the normal SpongeBob, but he's tiny, only has one tooth, and is much more square-shaped than the normal SpongeBob. He also jabbers in unintelligible gibberish rather than real speech.
Role in episode
When SpongeBob and Patrick are sad that they have to leave each other, they decide that they cut out parts of their body to make mini versions of themselves, and Patrick gets the SpongeBob mini. Throughout the episode, Patrick tortures Mini SpongeBob, so he and Mini Patrick torture SpongeBob and Patrick. Mini SpongeBob and Patrick then try to have a good living in SpongeBob's house, but they are too small, so SpongeBob and Patrick give mini SpongeBob and Patrick their own houses. Afterward, Squidward finds himself in the sand while Mini SpongeBob and Patrick are playing.