Our heroes are poor and/or outcasts. Potter himself is an odd, troubled kid brought up by a dysfunctional foster family. Grainger is a Muggle-born, a despised class within the magical community. The Weasleys are hard working and conscientious but poor.
The whole arc of the story (because each book essentially recaps THE SAME story) is of the triumph of the underdogs through unity and collective action.
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