Protagonist Cassie Sullivan is a typical teenager before the Arrival. She goes to high school, gossips with her best friend Lizbeth, and has a crush on the charming football player, Ben Parish. She is snarky and opinionated, strong and compassionate. Cassie argues with her father over his ideas about the aliens after the Arrival, her opinions leaning more on the violent side. As the first three waves come and pass, she becomes more vigilant over her father and younger brother Sammy. She still defers to her father’s decisions but has strong opinions of her own. When soldiers come to take the children of Camp Ashpit, Cassie is horrified at the idea of splitting up the family (especially after having lost her mother to the third wave). She instinctively knows that they need to stay together. However, her father insists that Sammy go, believing that he has a better chance of surviving if they follow the soldiers’ orders.

Cassie has good instincts but doesn’t necessarily follow them when she’s with her father. For this reason, she doesn’t run from Camp Ashpit the moment the soldiers arrive, a fact that likely saves her life. It is clear her father understands that showing their fear or attempting to run will only cause them to be killed sooner.