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- "You're human, you should be scared."
- — Teresa
In The Kill Order, a year after the Sun Flares, five-year-old Deedee, her parents, and older brother Ricky were living in a village with a large group of people. Two months before Mark's village was attacked, Deedee's village was attacked by the Post-Flares Coalition with infected darts, killing most of the people, including her parents. She was hit by a dart too, but survived. When the Flare later began to spread, everyone in her village, including Ricky, shunned and abandoned Deedee when they realized she was immune to the virus, believing her to be a demon of some sort.
Deedee was eventually found by Mark, Trina, Alec, and Lana, who decide to take her along and she forms a close bond with Trina. Deedee, Trina, and Lana are later found by some employees of the PFC and given back to the survivors of Deedee's village as part of self-preservation. The survivors, having now become Cranks, intend to sacrifice Deedee. Lana is brutally attacked and given a mercy kill by Alec while Deedee and Trina are saved. Mark, Trina, and Alec, now all infected with The Flare, decide to send Deedee to the PFC base in Alaska using a Flat Trans in the safe part of Asheville, hoping that her immunity could be used to find a cure. Mark gives Trina a note for the PFC disclosing her circumstances. Deedee tearfully bids Mark and Trina goodbye and makes it to the base through the Flat Trans. Mark, Trina, and Alec then sacrifice themselves to take out the PFC revolters.
At WICKED, Deedee is renamed "Teresa Agnes" after Mother Teresa. Two years later in the epilogue, a seven-year-old Teresa meets Thomas and is told the both of them will help build the Maze. Thomas and Teresa begin to bond over their pasts and Teresa tells him that her original name was Deedee.
Like Thomas and two other kids little Teresa is kept separate for years in the custody of WICKED. Frequent medical tests and daily classes dominate her daily routine, until she finally is allowed to meet Thomas. The two of them are elite subjects who are supposed to help building the Maze for the Maze Trials. One night a boy called Newt comes to take them to a secret meeting he and his friends Alby and Minho are having in the basement. They form a close friendship which comes to an abrupt end when Alby, Minho, and Newt along with the other subjects are about to be inserted to the Maze and feel betrayed by Thomas and Teresa. After this Teresa dedicates herself to her programming work while Thomas mostly watches his friends in the Maze. When more than a dozen high-ranking WICKED staff members including Chancellor Kevin Anderson get infected with the Flare, the immune elite subjects Teresa, Thomas, Aris and Rachel have to eliminate them (The Purge). Thomas and Rachel are feeling bad about this, but Teresa and Aris seem to accept the necessity easily. After being sent to the Scorch in order to test the long-range communication of their implants Thomas develops the plan to get inside the Maze and rescue his friends. He shares his thoughts with Teresa, who tells him she's in. She even convinces Dr. Ava Paige of the plan, and next day Thomas and Rachel are inserted to the Mazes A and B – though with their memories swiped, contrary to the original plan. Only Teresa and Aris, who follow one day later, will have their memories intact.
In The Maze Runner, the day after Thomas arrived in the Glade, Teresa showed up unconscious in the Box. This was a huge shock to the Gladers because Teresa was the first female to ever come to the Glade. At first she was believed to be dead by the other Gladers until she sat up and declared, "Everything is going to change," before falling back into an unconscious state. The Gladers found a message in her right fist from the Creators that read, "She's the last one. Ever." She spent the next several days in a coma, managing to contact Thomas telepathically a couple of times (thanks to the brain implant WICKED had given her). When Teresa finally woke up, she found that just before she came up the box, she had written 'WICKED is good' on her arm. She and Thomas had no idea what it could mean, but they figured it was important. The other Gladers didn't trust her, so they threw her into the Slammer. Teresa and Thomas formed a bond, realizing that they must have known each other before the Maze, and communicated with each other telepathically. When the Gladers escaped the Glade, Teresa, Thomas, and Chuck turned off the Grievers. After they were transported to a new building by WICKED, Teresa was put into a separate dormitory from the rest of the Gladers. She was last seen taken to the room, but later spoke telepathically with Thomas.
In The Scorch Trials, Teresa was moved to Group B by WICKED the next day while Thomas wakes up and is unable to find Teresa. WICKED threatens to kill Thomas unless Teresa pretends to betray him throughout the Scorch Trials. She cuts off their telepathic dialogue. After she has joined the girls of group B she captures Thomas, hurts and beats him and finally almost kills him, with Aris' help. This makes Thomas confused and hate Teresa as he doesn't know why she was doing this. It was revealed that Teresa and Aris could talk together using telepathy during Phase One in the Maze. Aris also has a part to play in this forced betrayal. At the end, Newt, Minho, and Thomas refuse to trust or forgive Teresa, even though she tells them she did it because she didn't want Thomas to die. Teresa becomes upset because of this.
In The Death Cure, Thomas still feels anger towards her, but still wants to be her friend. Rat Man gives the Gladers and the girls from Group B the option to get their memory back. Teresa immediately decides to get her memories back and tries to convince the others, but Thomas, Minho, and Newt refuse to trust her. She also says "WICKED is good".
It is revealed that she and the others broke out and forced a pilot to take them to Denver after they got their memories back. Thomas and his friends then learned that she had been captured by the Right Arm along with many other Immunes as part of a ploy to take down WICKED. The Right Arm reunites Teresa, Harriet, Sonya, and Aris with Thomas, Minho, Brenda, and Jorge. Thomas asks Teresa why she didn't try to break him, Minho, and Newt out as well when she made her escape, and she says she had heard that they had already escaped. (It's unknown how this rumor started.)
Later, after the Right Arm successfully infiltrates WICKED using the Immunes as an excuse to get inside, the hundreds of Immunes are placed in the Maze by WICKED for temporary safekeeping. Thomas, Teresa, Minho, Aris, Harriet, and Sonya find them. As they all escape, the Right Arm (who doesn't care about lives lost as long as WICKED is taken out) starts setting off bombs, triggering the collapse of the building. Then, when they make it back into the main headquarters of WICKED, Grievers activate, killing several people; one grabs Teresa and sucks her into its flesh, but Thomas then saves her. After a fight with WICKED members, as the surviving Immunes are running for the Flat Trans, a huge chunk of ceiling broken by the explosions falls directly towards Thomas and Teresa. Teresa pushes Thomas out of the way and gets crushed instead, only her face and arm able to be seen sticking out from beneath the huge stone. Stricken, Thomas tells her, "I'm so sorry," and Teresa tells him, "Me... too. I only ever... cared for..." Thomas is then pulled away by Minho and Brenda, leaving Teresa as she dies.
The Maze Runner
In the Maze Runner film, Teresa arrives some time after Thomas. Since the box appears to be out of schedule, the boys gather and Newt jumps down into the box to see what's this about. Teresa is delivered unconscious, making Newt comment: "It's a girl. And I think she's dead." But she briefly opens her eyes until she finds Thomas, saying his name. Then she goes back to coma again for some time. Later a gatherine is interrupted by Chuck because of the girl. She is now hiding on the platform, shouting and throwing stones at the boys to keep them away. Only Thomas is allowed to come up and speak with her. They wonder why they are different.
Soon after that Alby, who has been stung by a Griever, wakes up to the Changing. He starts fighting, but Teresa suddenly remembers she had two syringes with a serum with her, when she arrived in the Glade. She's able to inject one of them to Alby who instantly calms down.
Next there's a commotion because the doors don't close for the night. Without much time for the Gladers to prepare, Grievers enter the Glade and kill many of the boys. One Griever attacks Chuck, but Alby saves the younger boy by hacking off the stinging limb. One moment later Alby gets caught by the same Griever and dies. Thomas uses the stinging device to sting himself in order to regain some memories that might help them escape the Maze.
In the next morning, Gally, who has taken charge after the Griever Attack, has Teresa and Thomas tied to poles right at a door, as an offer for the Grievers. But his plan fails, Thomas and Teresa get free and convince most of the Gladers to come with them to finally escape. While the others fight against the Grievers, Teresa punches in the code and so opens the exit. The surviving Gladers find themselves in the WCKD headquarters, receive a video message from Dr. Ava Paige, see Minho kill Gally, who kills Chuck, and finally get rescued via helicopter.
The Scorch Trials
Contrary to the books Teresa is almost the whole time with the boys, from their escape from WCKD to the Camp of the Right Arm. In the beginning all of the surviving Gladers are brought to a WCKD facility buzzing with activity. They can take a shower and get medically tested. During these tests Teresa gets separated from the boys, sedated and tubed. Some time later Thomas and his new friend Aris persuade the others to escape from WCKD. They rescue Teresa and get out. Together they hide from the search patrols and helicopters. They find an abandoned mall where they stock up clothing and nutrition, but also awake their first Cranks. Two boys of their group die, but most of them manage to escape. Winston receives a nasty scratch at his belly, thus he gets infected with the Flare. He gets weaker as they proceed through a desert, and finally collapses. He reveals his wound to the others and asks them to end it. Newt hands him a pistol, and each of them takes their leave from their comrade.
Some time after when they pass the night without shelter in the desert a heavy storm builds up and they can just in time make it to the first building of a ruined city. When they turn on the lights, chained-up Cranks reach out for them, screaming. A shorthaired tough girl comes to take them to the boss – Brenda presenting the group to Jorge. He wonders how their arrival could be his advantage and has them hanged up by their feet until they tell him everything. In an unattended moment they manage to free themselves. But then WCKD forces storm the place in search for the fugitives. Teresa and most of their group escape with Jorge, but Thomas and Brenda get separated and have to find their own way as the building explodes.
They find and rescue Thomas and Brenda who have been drugged out on a party by Marcus who was supposed to assist them but had switched sides and now meant to sell them to WCKD. Jorge forces the man to tell them where they can find the Right Arm. The group goes to the mountains in Marcus' car.
They have to stop at a roadblock, and when they get out of the car they get immediately shot at by unseen people. None of them gets injured, though, and finally two girls (Harriet and Sonya from Group B who now work for the Right Arm) round them up and threaten them with their guns. They recognize Aris, saying they were in the same Maze together. They take all of them to their leader, Vince.
Finally everyone seems to enjoy the peace of the moment, former Gladers and Group B each relaxing with their peers, Teresa seeks solitude on a mountain. Thomas follows her and she tells him about her parents – her Crank father finally past the Gone, her mother infected, too. She tells him about her resolution to find a cure and her infinite trust to WCKD. At this moment WCKD bergs show up in the sky, and Thomas concludes that Teresa has called them.
After a desperate fight the Right Arm is defeated and WCKD has them all kneel in lines. Teresa stands at the side of Chancellor Paige with a sad face – she had hoped that Thomas would see reason, would understand and support her. Nearly all captives are brought into the berg, including Minho. Teresa is seen with the WCKD personnel (Paige, Janson). Thomas announces that he will find and rescue Minho. Vince and the remaining group agree.
The Death Cure
Teresa is back working as a scientist for WCKD. She is seen in a white lab coat searching for the cure.
Minho is being tortured, sensors measuring his killzone activity. Teresa never intervenes, no matter what Minho suffers. She then goes to see a little infected girl, injecting her the content of a syringe. She clearly hopes that she has finally found the cure. The girl gets better. Dr. Paige watches them from the corridor through a window. They are talking and laughing.
Once she tries to talk to Minho, but he shows no reaction at all. He seems to be broken. When Teresa is about to end the meeting, Minho attacks her. Guards take him away. Teresa, still shocked by that incident, walks along the corridor, when Dr. Paige finds her and urges her to come and see the little girl. She has gotten worse than ever before, almost fully crank.
When she leaves the WCKD building she suddenly recognizes Thomas on the other side of the street. She follows him into an alley and they talk. Teresa is happy to see him. Thomas asks her if she is sorry for what she has done to them. She says she is, but she would do it again, because she thinks it is the right thing to do. From behind Gally puts a sack over her head and brings her to an abandoned church to meet the Gladers along with Brenda and Jorge. Gally questions her and forces her to help them. Reluctantly, she gives away informations like how many immunes they keep at WCKD, where they are and how to get to them. She also has to remove the implants from their necks to prevent further detection. With shock and surprise she realizes that Brenda looks good, really healthy. She presumes that Brenda has been treated with the proto-cure regularly, but they tell her that her last treatment was back in the mountain camp of the Right Arm. Teresa realizes that Thomas's blood must have a hitherto unknown healing effect, and she takes a sample. She is forced to accompany Thomas, Newt and Gally into the WCKD building, opening all the doors for them.
While Gally brings the Immunes and the serum to Brenda who is waiting with a getaway bus, Teresa, Thomas, and Newt have to find Minho in the medical section. Janson gets on the lift they are taking, talking with Teresa, unaware that the guards behind them are Newt and Thomas. When Janson gets off the lift, Teresa follows him. Only this moment he realizes Thomas's presence, but Teresa closes and seals the lift, allowing them to go on on their own accord. They find Minho and fight their way out of the building.
Meanwhile, Teresa examines Thomas's blood sample and finds that his blood does not only keep the Flare virus in check but indeed destroys it. This is the cure they have been searching for. She broadcasts her voice all through the Last City to persuade Thomas to get back to her and make the cure and save Newt and so many others.
After Newt's death, Thomas comes back to WICKED. He encounters Dr. Ava Paige, but before she can give him any information she is shot by Janson who takes Thomas to a lab where Teresa takes his blood for the cure. There Janson reveals that he needs the cure for himself, him being infected, too. And he plans to control the commercialisation of the cure. He wants to determine who is worthy to get cured. In his plans Thomas serves as the one donating as much blood as can be taken from him. Teresa listenes in horror – this was not what she fought for. She has sacrificed a lot, but it was for healing people in need, not for the wicked aims Janson has in mind.
In the following fight Thomas gets shot in the stomach but manages to throw a weight, too. Janson thinks he missed, but then becomes aware that the weight hit the glass front that separated them from the Cranks behind the barrier. They swarm him and he has no chance to survive.
Teresa and Thomas make it to the roof. Jorge and the rest of their group come with a berg to rescue them. Jorge tries his best to keep the hovering berg close enough to the roof top for them to jump on board, but wounded Thomas is too weak. Only with Teresa's help his friends manage to pull him aboard. Then a burning building crashes onto the WCKD building, and Teresa falls to her death.
Physical Appearance and Personality
Teresa was very beautiful, and looked about fifteen years old, though actually seventeen. She was described in The Maze Runner as being "thin, but not too small," "roughly five and a half feet," with "burning blue eyes" and skin that was "pale, white as pearls." She had silky "tar black hair" with "long legs" and "pink lips." She is highly intelligent and quite tough.