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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. Deedee is given a note by Mark which states she is immune. She tearfully bids Mark and Trina goodbye and makes it to the base through the Flat Trans while Mark, Trina and Alec 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.
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, making 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.
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."