Who is Cassie Parrish from Wintergirls and what is their importance?

Cassie Parrish is Lia's ex-best friend. At the age of eighteen and at the opening of the novel, Cassie dies, alone in a motel room. Before she died, Cassie tried to call Lia 33 times, but Lia never picked up the phone. Throughout the novel, Cassie is a ghost, haunting Lia, urging her to finish herself off and join her on the other side.