OK, bear with us on this one. As we mention in the "Setting" section, Harlem is almost like another character in the story, and it's not a positive one. Both Sonny and the narrator recognize Harlem as the cause of many problems for many people. The bleakness of Harlem is part of what Sonny tries to escape by doing drugs, and even as kids Sonny and the narrator hear their mother lament that they don't live somewhere safer. Harlem sort of looms as a villainous presence throughout the story, and we think that's what makes the neighborhood the story's antagonist.