What do we learn about Shannon, Izzy's bully, over the course of the novel, What She Left Behind?

Over the course of the novel, we discover that Shannon was sexually abused and that her father abandoned her family, hence the source of much of her acting out. Shannon acts as a reverse doppelganger of Izzy's, in that both young women have suffered abuse at the hands of their fathers. But whereas Izzy has drawn deep within herself and cuts herself to deal with her pain, Shannon instead externalizes her pain and tries to inflict it on others.