Mac is a talented piano player, about Thomas's age. Thomas mistrusts him at first, but the more he sees him, the more he wants to be his friend. Mac is interesting because, like Mayhew, he's trying to step out of the cycle of feuding that his father and brothers are involved in. He doesn't participate in trying to scare the Smalls. Unlike Mayhew, Mac manages to step out of the situation without leaving town.
It's not clear what Mac's home life is like, but it would have to be a little weird, what with his father and brothers "always digging up trees and putting them back" (11.96). According to Mr. Carr, the Darrows even moved their house once, so they could look for treasure underneath it.
Plus, Pesty says that Mac "got slapped down" (15.80) for refusing to help vandalize the Smalls' kitchen. That doesn't sound good. It makes us think that Mac and Pesty might not have an ideal home life. Notice, though, that it's Mac's older brother who slaps him, not his father or mother. Mayhew suggests that Mac will get teased by the townspeople, because of the scaring doled out to his father and brothers. As a result, it might take some time before he and Thomas strike up a friendship. But by the end of the story, we have the feeling that he and Thomas will become good friends.