Gaia is the child of Caine and Diana, born during Fear. Her body was taken over by the Gaiaphage shortly after her birth. She is described as having dark blue eyes and hair like Diana's and is said to look very beautiful, just like her mother.
Gaia was first seen during Plague, with Diana realizing she was pregnant at the end of the book by accidentally reading her baby's power level (2 bars at the time). Due to her pregnancy, Diana left for Lake Tramonto instead of staying in Perdido Beach with Caine, as she decided her child was now her main priority.
The Gaiaphage found out about Gaia and sent Drake to get Diana so the Gaiaphage could be reborn. At this time, the Gaiaphage was growing weaker and the barrier was growing darker. Drake captured Diana and Justin at Lake Tramonto and made his way to the mineshaft. When they reached there, Penny and Brianna were there traveling in the darkness (the barrier had grown fully dark so there was no light). Penny followed Drake and created an illusion of lava to distract Brianna. Inside the mineshaft, in the deep, central, slope area - created by Duck - Diana was going into labor and Drake nor Penny knew what to do. Brianna came along and shot Penny and Drake had already turned into Brittney and her head was cut off by Brianna. When Gaia was born, the Gaiaphage was disguised as a crib and Brianna laid Gaia in it. At that instance, the baby was taken over by the Gaiaphage.
Because Gaia started to grow with supernatural speed in the time period between Fear and Light, she had already reached an age equivalent of at least 5–6 years by the beginning of Light. Now growing ever stronger, Gaia continued to become more and more dangerous to her surroundings. In the beginning of Light, Gaia is quite innocent, having not used her powers to cause any serious damage to the FAYZ. But that ended when Alex, a red-haired man from the outside world, was transported into the FAYZ Wall.
Gaia immediately decided it was time for her to get her revenge, and set out on a journey throughout the FAYZ to kill every living human in the area. During an encounter on the highway, Gaia cripples Sam and breaks Caine's spirit by torturing him. She leaves Sam on the road and takes Caine along with her. In an ambush by Edilio, Gaia kills Brianna and proceeds to destroy the town. When she loses Jack's power after his death, she flees to the border, killing children and adults through the FAYZ wall. After Orc is killed, she turns to face Caine, who has been possessed by Little Pete. They kill each other in a dramatic showdown and when Gaia dies the FAYZ comes down.
Because she is being controlled by the Gaiaphage, she is able to use powers from some of the children in Perdido Beach. She is able to use telekinesis from Caine Soren, light/fire from Sam, gravity cancellation from Dekka, super speed from Brianna, superhuman strength from Computer Jack, camouflage from Bug and healing from Lana, and near immortality from Brittney.
Gaia means "Earth" or "World" and is used as a primordial deity in Greek mythology.