She used to be a sweet, innocent, Daddy's girl. In the early pages of the novel, Cassie says she changed. Being a survivor turned her into a killer. If survivors want to survive, they have to kill. That helps her in some ways, but also makes her person vs self conflicts worsen.
That is the one word that I would use to describe her. Cassie Sullivan is the protagonist of The 5th Wave and proves again and again that she is a survivor.
