Teresa

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.