Sophia had a rough childhood even before the zombie apocalypse, suffering occasional abuse from her father. When she and her parents made their way to Atlanta during the initial outbreak, they encountered Shane Walsh and Lori and Carl Grimes. Together they joined up with other survivors who fled the city before it got bombed by the military, setting up a camp near an abandoned quarry. She and Carl became good friends during the time they spent at the camp.
Disappearance and deathIn the Season 2 episode "What Lies Ahead", Sophia is pursued off the highway by two zombies. She runs into the woods and Rick Grimes goes after her. Rick catches up to Sophia and tells her to hide near a small creek while he lures the walkers away. Rick deals with the walkers but when he returns to the creek he finds Sophia is gone.
Sophia remains missing for several episodes but Rick and the others continue to search for her. In the episode "Pretty Much Dead Already", the group discover to their horror that Sophia has turned into a zombie and was among the group of undead locked away in Hershel Greene's barn. Evidently, Sophia had been bitten by a walker out in the woods and had been found and snared by Otis before his death. When Rick and co. open the barn and kill off the zombies, Sophia is the last to emerge and it is Rick who pulls the trigger on her.