Bart's guilt

Bart's Guilt is an inner demon of Bart Simpson's, who personifies his guilt. He appeared in the episode The Cad and the Hat.

Biography
After Bart threw away Lisa's hat, he felt guilt for the first time and was visited by a humanized version of his guilt, which grew as he felt more guilt. When Lisa forgave Bart, his guilt shrunk, got knocked out by an airplane, and landed in the Springfield Nuclear Power Plant, where it mutated into a giant monster, much to the enjoyment of Mr. Burns.