Cassie Sullivan is the main character in the young adult science fiction novel series, The 5th Wave, written by Rick Yancey. She is a teenage girl who survives an alien invasion that devastates Earth. Throughout the series, Cassie demonstrates resilience and determination as she navigates a world filled with danger and uncertainty. In the story, Cassie is on a mission to find her younger brother, Sam, who has been taken by the aliens. As she encounters other survivors, she learns to trust and work with them, forming alliances that are crucial for survival. Cassie's journey highlights themes of love, sacrifice, and the struggle for humanity in the face of overwhelming odds.