Cassie is one of Grady Tucker's neighbors. She's first mentioned when Grady meets Will Blake, who describes her as "kind of weird". When Grady's family settles into their new home in Florida, strange things begin to happen in the swamp nearby. Eerie howls can be heard in the middle of the night, and worst of all, the local wildlife is being mutilated by an unknown creature.
All blame falls on Grady's new dog, Wolf as the culprit. Grady, however, is determined something else is to blame. When Grady finally meets Cassie in person, she's the first to tell him the legend of a werewolf living in the swamp. Will mocks her story, but she truly believes in the legend. She tells Grady the hermit living in the swamp is the werewolf, and Grady starts to wonder if this could be true.
Grady starts to believe Cassie's story and eventually runs into the werewolf. However, the creature is really Will, not the hermit. Wolf attacks Will, but not before he bites Grady and passes on his curse. Grady and Cassie remain friends, with her joining him and Wolf on hunts. She appears to still be on constant lookout for werewolves.
Cassie firmly believes in the werewolf legend. Will is constantly teasing and discrediting her stories, likely to hide his own secret. Despite being labeled weird, almost everything she tells Grady about the werewolf is true. At the end of the book, she accompanies Grady and Wolf in exploring the swamp. Grady says she's fun to be around, but wishes she would drop the subject of werewolves.
Cassie is has cat-like green eyes and freckles covering her face. She has rust colored red hair, which she wears in a ponytail.
- Cassie is omitted from the television adaptation of the book, leaving Will as the one to tell Grady about the werewolf legend.