Cassiopeia Marie "Cassie" Sullivan is the main character of The 5th Wave trilogy by Rick Yancey. She was a survivor of the 'waves' sent down by the Others to wipe out humanity. Her younger brother is the only other surviving member of their family as she has lost both of her parents to the waves.

Cassie has joined together with Squad 53 and travels alongside them as they try to fight against The 5th Wave that has just been started by The Others to eliminate what is left of humanity. 