The 5th wave, fiction

I have not yet finished the book but so far what I've read is the main character Cassie has changed. In the very beginning Cassie is mad about the beginning of the invasion. Cassie's brother is then taken by the aliens later in the book and its seems like Cassie's attitude changes. She goes from a scared angry girl afraid of the invasion to a determined, mentally stronger young women who will do anything to get her brother back.  Cassie definitely matures as the book goes on.