Literally nobody knows where Tess came from. She suddenly was seen wandering in Meridian, totally naked, when they found her. When talking to her she kept talking about something terrible heading this way, a force of nature that will bring doom to all of them. Tess was quickly deemed as an insane person and put into the mental asylum.
While Tess is a kind person, no one can really see past her insanity. She also seems to suffer from multiple personality disorder. This can be taken from that fact that she sometimes can be very serious and actually tries to explain the situation and demanding a release, only for people to not take it serious. But the majority of the time she just is a spastic lunatic.