Theresa Cullen is a senior manager in the company that runs Westworld, she is the Head of Quality Assurance, responsible for Westworld's standards and safety.
Theresa tells Bernard that one of the Hosts in storage is acting strangely. She warns him that with every update they upload to the Hosts, bugs might surface that could lead to the destruction of Westworld. She says they are overdue for a critical failure since the last happened 30 years ago. After an android malfunctions, she insists that Bernard run diagnostics to see what the problem is. Theresa is concerned by the fact that the Hosts can improvise more now with the most recent update. She orders the recall of Hosts who have been updated. To appease the Guests, she makes the saloon heist especially gruesome.
Theresa checks in with Bernard to make sure that all the Hosts are running smoothly. She tells Lee that he is only getting 20 additional hosts to add to his new narrative. Theresa goes to Bernard's apartment and the two proceed to sleep together.
Theresa warns Bernard about how the board isn't sure about Ford's new narrative. She says that Ford's new storyline is already conflicting with pre-existing narratives. She also asks Bernard why Hosts are still being pulled and if he's keeping any problems from her.
Theresa's team takes over the investigation of the wandering Host. Theresa meets with Ford at a restaurant in Westworld that she visited as a kid. She informs him that a board representative would be arriving soon to see the new narrative. Ford warns Theresa to be careful with Bernard, confirming that he knows that they are sleeping together.
Theresa tells Bernard that Ford knows about their relationship. She breaks up with him as a precautionary measure. She visits Lee at the Mesa bar and asks him to come back to work. He refuses so she leaves but not before telling him to sober up. Bernard shows up to Theresa's apartment to talk about the anomalies in the park. But before he can Elsie warns him that Theresa is responsible for the information leak.
Theresa, along with Charlotte Hale, devise a plan to prove Ford's edits to the Hosts are making them dangerous. They do so by resetting Clementine Pennyfeather and thus shorting out her key response functions. Hale fires Bernard after showing the effects to the board and seeds doubt as to whether Ford can be of any more use to the company. Bernard, worried about Elsie, confronts Theresa about both the hidden theatre and them sneaking information from the Hosts. Theresa explains that they only were doing so in the case that Ford would wipe out the Hosts' memories upon his "retirement" from the company. Bernard leads Theresa to the theatre where they find a hidden lab and blueprints for both Dolores and Bernard himself. Ford reveals himself and has Bernard kill Theresa to prevent her from ruining his life's work.
Theresa is a very smart, calculating woman. She was clever enough to plan ahead in the event of Ford leaving and wiping the Hosts' memories, and also had the foresight to end her relationship with Bernard as a precaution after Ford found out. She also hatched a plan with Charlotte to get the board to oust Ford.
She doesn't appear to have a stomach for violence, as in Trompe L'Oeil she hastily ended the part of the demonstration where Clementine Pennyfeather was beaten by a male Host, turning away from the sight.
She isn't brave in the face of danger, as when Ford ordered Bernard to kill her she begged him not to and made no attempt to run away or fight back.
Theresa was in a clandestine relationship with Bernard Lowe, however, she broke it off after Ford found out about it. She appears to trust him very much, as shown by when she willingly followed him into an unknown section of the park despite him having just revealed that he knew she was transmitting information outside the park.
Theresa doesn't appear to like Lee Sizemore.