Theresa was introduced to Faraday's mother Eloise Hawking at Daniel's graduate commencement, but Ms. Hawking was dismissive of Theresa and her son's relationship with her, preferring that her son focus only on his research work. ("The Variable")
Though she survived (unlike the rat Eloise), Theresa was chronically affected and became an invalid, requiring constant home care by her sister, Abigail Spencer, and a nurse. During her occasional moments of lucidity, she described being a young child or having just been with relatives long deceased. Having previously funded Faraday's research, Charles Widmore financially provided for Theresa's care. ("Jughead")
Because of what happened to Theresa, Faraday fled Oxford for the United States, and the university expunged any reference to him from their records. When he abandoned his laboratory, he left behind on the floor a photograph of the two posed together on the Oxford University campus.
In the casting call she was described as "Female, 30s, British, blonde. Pretty suffers from dementia but also has short periods of complete lucidity."
Theresa was first confirmed to be Daniel Faraday's girlfriend in the April 23, 2009 official podcast.
Despite a lack of awareness of where/when she is, Theresa appears to handle the temporal displacement much better than others as she was revealed to still be alive in 2007.