Mars Bar Thompson is a young black boy who lives in the East End. He is a dynamic character. He changes during the course of the novel due to his experiences and actions. At first, Spinelli describes Mars Bar as a stereotypical tough-acting, not-afraid-of-anything bully. Mars Bar harasses Maniac by blocking his way and tries to intimidate Maniac with an intense glare. Mars Bar uses words spoken in the East End such as "bad" and "fishbelly." Mars Bar walks with a "super-slow dip-stride slumpshuffle," which is said to stop traffic because it takes him so long to cross the street.
Mars Bar dislikes Maniac because Maniac is white. Maniac confuses him because Maniac is not prejudiced toward him or afraid of him. When Maniac takes a bite of Mars Bar's candy — an unheard of act because blacks and whites never put their mouths where the other has been — Mars Bar is shocked by Maniac's nerve. Later, Spinelli implies that Mars Bar put the racial epithet on the Beales' house because he'd previously called Maniac "fishbelly" and because he is prejudiced against whites. As Maniac leaves town, Mars Bar and his group of friends heckle him.
Mars Bar is as ignorant of white people as John McNab is of black people. Mars Bar challenges Maniac to a race and loses. Because Maniac humiliates him by showing him up, Mars Bars' hatred is directed at Maniac rather than at the white race. Mars Bar begins to change when he can finally see Maniac as an individual. Afterward, when Maniac goads Mars Bar into being the "baddest" by crossing Hector Street and going to the West End, Mars Bar feels proud because his fame has spread to the West End. The Pickwell children know who he is and are respectful of him. The McNabs, on the other hand, are disrespectful. While Mars Bar is at the McNab house, Mars Bar is quite uncomfortable. "His jaw [is] clenched and his eyes [keep] straying to the gaping hole in the ceiling — and to the Cobras. . . ." Mars Bar is "as scared as any normal kid would be, but [is] not showing it . . ." Underneath Mars Bar's bravado, he is vulnerable just like anyone else. Crossing Hector Street with Maniac earns him Maniac's respect — not for what he does, but for the person he is.
Mars Bar begins running in the early mornings like Maniac. Soon they are running together, never talking or paying attention to the other. Mars Bar and Maniac become equals. Mars Bar forgets color, rescues Russell McNab from the trolley trestle, and takes the McNab brothers back home — to the West End. Mars Bar shows his concern and love for Maniac when he finds him sleeping in the buffalo pen at the zoo. Mars Bar even invites Maniac to live with him and his family. Spinelli creates the friendship between Mars Bar and Maniac to portray the beginning of desegregation in Two Mills.
