Theresa is "The Hero's" older sister who has prophetic powers due to her ancient bloodline which she shares with her brother and his descendants. During the bandit raid of Oakvale, Jack of Blades (the game's antagonist) blinds her. After stumbling through the forest she is found by Twinblade's men. Due to her powers she eventually becomes second in command. Despite being blind Theresa has extrasensory perception, also a result of her lineage. Once the hero (her brother) defeats Twinblade she leaves to find her mother. While looking for her mother she is captured by Jack of Blades. When the hero defeats Jack of Blades in the final battle, Theresa tells the hero that he has a choice: "Strike me down now with the Sword of Aeons and gain the power Jack dreamed of, or cast it into the vortex and rid the world of its evil forever".
If you decide not to kill her Theresa heads off to the mountains and is taken in by a nomad who later tries to steal her powers. Theresa kills him, moves off to live in the east and is not seen for 500 years.
At the start of Fable II, the player meets an old woman named Theresa. While it is never stated in the game, she is largely believed to be the same Theresa as from Fable I. In Fable II, Theresa is once again a very powerful will user, she appears occasionally throughout the storyline to assist the Hero. During the introductory section of Fable II, she convinces Rose (the main character's sister) to buy a music box which leads to Rose's demise. After the death of Rose, Theresa adopts the critically injured hero and becomes their guide for the rest of the game. The only time she will make an actual appearance, however, is when she recruits another Hero who she will bring back to the destroyed Heroes' Guild. Theresa also translates Lucien's diary so the Hero can gain entry into the Spire. When asked how she could read the diary if she is blind, she claims that she could look into worlds other than their own. After she finally assembles the four heroes, she disappears until Lucien is defeated. Then Theresa gives the player a choice between reviving everyone who's died from the spire's construction (specifically the hero's loved ones) or giving him a large sum of money. Regardless of what choice the player makes, Theresa gives the player the reward of his choice and then ominously states: "But the spire is...mine. Begone."
Theresa appears once more in Fable II in the See the Future DLC. There, she reveals that the fate of Albion and the then unheard of City of Aurora will rest on the shoulders of the Hero's children.
Theresa's role in Fable III is much the same as it is in Fable II, though much smaller. Theresa guide you on the Road to Rule, the mystical equivalent to a character progression menu. In Fable III, Theresa does not act as a mentor like she does in Fable II, that role is take by Sir Walter Beck.
Theresa's role in Fable: The Journey is mostly unknown. What is known is that Theresa is on the run from a darkness that is in some way related to the Crawler from Fable III. Theresa has lost her powers and joins up with the main character of the new game as the ride across Albion.