Hope is the protagonist of Hope was Here, so the antagonist (or one who works against her) could be one of many characters in the book.  The case could be made that her mother is her antagonist since she abandons Hope in order to live her own life.  This puts Hope in a position of dependence on her aunt, Addie.  Another antagonist could be Millstone, who is working against Hope's (and other characters') goal of electing GT mayor.  In order to answer this question, ask the question, who is standing in the way of Hope achieving her goals?  In what way does this person create conflict for Hope. Of course in this story, Hope ultimately achieves her goals of finding a family and a home despite the work of the antagonists.