I can think of quite a few "A" words that would describe Maniac Magee. Awesome, athletic, able, alienated, and altruistic.
Maniac is definitely an able bodied athlete. Early in the story he catches a football one handed then punts it directly back into the hands of the quarterback.
When the kid got down to the soccer field, he turned and punted the ball. It sailed back over the up-looking gym classers, spiraling more perfectly than anything Brian Denehy had ever thrown, and landed in the outstretched hands of still stunned Hands Down. Then the kid ran off. There was one other thing, something that all of them saw but no one believed until they compared notes after school that day: up until the punt, the kid had done everything with one hand. He had to, because in his other hand was a book.
Maniac continues his athletic prowess by hitting every single pitch that John McNab can throw. He even successfully bunted the frog ball that McNab threw. Maniac then managed to make his bunted frog into a home run hit.
And that's how Jeffrey Magee knocked the world's first frog ball for a four-bagger.
I think Maniac is an altruistic character as well, because he is constantly giving up his time in a selfless manner for other people's benefit. For example, Maniac takes time to teach Grayson how to read. Another example of Maniac's altruistic spirit is his care for the McNab family. Maniac winds up being a guardian angel of sorts over Piper and Russel McNab.
Lastly, Maniac is an alienated child. For as "cool" and capable as he is, Maniac doesn't have a home. He ran away from his Aunt and Uncle. Two Mills is a divided town by race, and Maniac can't quite seem to fit in with anybody. He likes living with the Beale family, but he's an outsider, because he's white. Maniac doesn't fit in with the other families either though, and consequently Maniac is frequently an outsider in the story.