The Question and Answer section for Maniac Magee is a great resource to ask questions, find answers, and discuss the novel.
After his parents died in a car accident, Maniac was sent to live with his aunt and uncle. Maniac does, however, run away from their home to the novel's main setting in Two Mills, Pennsylvania.
What chapter are you referring to?
The book's antagonist, at least until the end. Mars Bars Thompson is a big bully who lives in East End and dislikes Jeffery because of his skin color and the fact that he beats him in a race.