The "problem" with all of these characters is that they are not really fleshed out as individuals but rather represent types. Characters portrayed in this miminalist kind of way are called "stock characters," which are very close to stereotypes. They are instrumental in keeping the story line going or are there for simple contrast to another more developed character, but as far as their particular function or role goes, that's about it.
In this highly entertaining if not very credible tale, all the characters represent types, polarizing into the opposing black and white communities. Amanda and Mars Bar represent the black community (she's the "straight" scolastic one ;he's the "wooley") and John McNab, the white (also rather marginal, along with his brother). "Maniac McGee" serves as the bridge between the two in both a literal and metaphorical way. (In fact, that is ultimately the whole point of the story.)
Besides racism, other ideas of prejudice against minority groups are also addressed, particularly when Mcgee is "adopted" by a senior citizen and they take up residence together in the mens' locker room at a local school. It is ironic that Mcgee, the eternal "outsider," is the one always bringing people together.
For more details concerning these characters and their resepective roles, click on the following references.
Maniac Magee is a young boy who is a fast runner. He is an orphan whose parents died when he was around 8. He is an accepting person who does not allow a person's race to influence his opinion of them. He is also a kind boy around 12 years old who is searching for a family connection that he did not have when he was living with his aunt and uncle.
Amanda is youg black girl who lives with her family. She likes to read and spends time carrying her books around. She is kind and cares about others, a quality which she displays after having her family meet and care for Maniac.
Mars Bars is a black youth who likes to run and play ball. He is distrusting of Maniac probably because of Maniac's race. He questions why Maniac is in his neighborhood. He is athletic and proud, and courageous. He attends a birthday party at a white child's house after Maniac's invitation.
John McNab is a young white boy who is arrogant and defensive. He becomes jealous when Maniac defeats his fastball by getting a hit. He is a leader who is imitated and followed by boys in his group.