Lisa Sullivan is the wife of Oliver Sullivan and the mother of Cassie and Sam Sullivan. She had been one of the victims of the 3rd Wave and had died from the plague. Soon her husband became lost in the 4th Wave leaving their children all alone in this new world. Her daughter sacrificed herself to stop the execution of the 5th Wave leaving her son as the sole surviving member of the Sullivan Family.
Lisa was a bit thrown when The Others appeared on Earth. She didn't seem to have any strong feelings about them until the 1st Wave struck and she had to pick up Sam from school while her husband went to get Cassie. Her husband thought that it would be wise to stay home, in relative safety, even after they learned that the 2nd Wave struck and flooded the coastlines. When the 3rd Wave struck, Lisa had gotten sick from the plague. She slowly was chipped away in front of her family. In the final stages of her illness, Cassie didn't recognize her. It was obvious that she would die soon and her husband thought it would be best to finally leave but he refused to until his wife passed so that she could be buried properly.When Lisa finally succumbed to her illness, she was buried in their house's backyard with a marked grave.
Cassie became broken hearted when Sam told her that he couldn't remember their mother's face before she became sick. When Sam had shot and killed Constance, he said that he gotten a flash and remembered what she looked like to Cassie's shock.