Teresa

"You're human, you should be scared."

— 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. When she finally woke up, she wrote on her arm, 'WICKED is good'. The Gladers didn't trust her, so they threw her into the Slammer. Teresa and Thomas formed a bond and communicated with each other telepathically. When the Gladers escaped the Glade, she, along with 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.

Thomas

"I promised him I'd save him, take him home, I promised him"

— Thomas, talking to Teresa while mourning over Chuck's Death.

In The Maze Runner, Thomas wakes up in a lift with no memory except for his name. When the lift opens, he is surrounded by other boys who also have no memory except their names. This place is called the Glade. The Glade is surrounded by a maze with stone walls. Every morning, the doors to the maze open, and every night they close. The Gladers do not want to be stuck in the maze when the walls close, for they could be "stung" by hideous monsters called Grievers. As Thomas adjusts to life in the Glade, he befriends a younger boy named Chuck.
When Thomas discovers that Runners go into the Maze to try to find a way out, he feels that he should be a Runner. Then, when a girl comes out of the Box, Alby and Newt call Thomas over because they feel that the girl and Thomas have a connection. Thomas denies having any memory of her, but he does feel like he knows her. The girl is in a coma and the Med-jacks take care of her.
At night, just as the walls begin to close, Thomas sees Minho dragging Alby towards the Glade, because Alby has been stung by a Griever and is unable to run. Thomas wants to go and help, but the other Gladers tell him it is too late. Right before the walls close, Thomas steps into the Maze. Minho is angry at Thomas for not staying in the Glade, as he believes they will all die now. He leaves Alby by the walls and runs off. Then Thomas hears a Griever, so he drags Alby to the wall and starts tying him to the vines, working him higher and higher above the ground. Thomas lures the Griever away from Alby by running deeper into the Maze. He later meets up with Minho and the Grievers chase them. Minho tells Thomas to follow him and they go to the Cliff. The Grievers are still chasing them, but Minho has an idea based on what Thomas did with the wall vines. They both stand at the edge of the Cliff, and when the Grievers charge at them, they jump away and the Grievers fall over the edge. In the Cliff, there is a Griever Hole, where things in the hole disappear.
In the morning, Thomas takes Alby off of the wall vines. Thomas, Alby, and Minho are the first people to survive in the Maze for a whole night. Alby is given an antidote for his Griever sting. When Thomas visits him, Alby tries to tell Thomas what he saw during his Changing. However, Alby grabs his own neck and starts choking himself. Thomas calls for Newt, and together they get Alby to stop hurting himself. Newt says that Alby is crazy, even though Alby says that he was being controlled. Later, Thomas is elected Keeper of the Runners, taking Minho's place. When Thomas visits the girl, she speaks in his head. He learns that her name is Teresa. In time, Thomas discovers that Teresa triggered the End. No one knows what this means, but soon the walls do not close and bad things begin to happen.
When Teresa is fully recovered, she rushes to Thomas. They talk and Tom develops a small feeling for her.
With the walls no longer closing, Grievers begin to take one kid each night.
Thomas and Teresa eventually figure out a code to the Maze. Thomas gets stung purposely by a Griever so he can remember his life before the Maze. He learns that there is only one way of escape: through the Griever Hole. The Gladers make it through the Griever Hole, losing half of their number, including Alby. They are greeted by someone who works for WICKED and Gally. Gally takes a knife and throws it at Thomas, but Chuck bravely takes the hit by jumping in front of Thomas. Thomas feels guilty because he had promised Chuck to get him out of the Maze safely. Then a rescue team comes in and seemingly kill the WICKED member, rescuing the Gladers.

Gally

"Everything started changing the moment you showed up."

— Gally

In The Maze Runner, according to Winston, Gally would have recovered a part of his memories after having been stung by a Griever in the middle of the day near the West door. Gally became a Glader after arriving in the Glade. However, he was despised by most of the Gladers due to his arrogant nature. Two years later, Gally witnessed the arrival of a teenage boy named Thomas who Gally immediately became enemies with.
After being interrogated with Thomas by Newt, Minho got into an argument with Gally that ended with Gally running off into the Maze and going missing. After missing for many days, Gally eventually returned to the Glade, scared and dirty, and warned the Gladers that the Maze's creators were going to kill them and that Grievers will take one of them every night. Plagued by a nameless terror, he snatches planks barricading windows and hit Newt with one of them. Thus a Griever manages to penetrate in the farm and seizes Gally before taking him in the Labyrinth.
Gally was apprehended by WICKED and taken into their control. When the surviving Gladers made it through the Griever hole, Gally emerged from the shadows with a knife and while under control of WICKED, attempted to kill Thomas. A young Glader named Chuck jumped in front of the knife and was killed in Thomas's place. Thomas, angered by Chuck's demise, attacked Gally. While Gally and Thomas fought, a group of adults entered the chamber and killed the WICKED agents. Thomas knocked Gally unconscious and fled with Teresa and the other Gladers.
In the film, Minho presumably killed Gally with a spear, so his appearance in The Death Cure in the films is questionable. Although, Gally was shown breathing after the spear wound. So it's highly likely he survived.

Minho

"It's kind of hard to ask a dead guy what he did wrong."

— Minho

In The Maze Runner, when Minho first appears, he is found by Thomas laying on the ground from exhaustion as he'd just gotten back to the Homestead from the Maze. Thomas calls for Alby and is directed by Minho to get him water. Alby telling Thomas that Minho is the only one who can boss around him like that. After Alby brings the water, Minho explains that he found a dead griever. The next day, Alby and Minho go into the Maze to find the dead griever, but, when Alby touches the griever, it springs to life and stings him. Minho has to carry Alby back to the other Gladers for help, but after the doors start closing everyone knows it is too late for them. Thomas jumps to the rescue a second before the doors close. Minho runs in panic, but Thomas, being the new guy, thinks there is hope. He climbs the wall and drags Alby with him. After a griever starts following them, Thomas flees, leaving Alby behind, then Thomas somehow fools the grievers which Minho sees. Minho grabs Thomas and together they trick the grievers over the Cliff. Then, the Gladers wake up to find that the three are still alive and have a Gathering to decide what is to be done with Thomas. Minho shows up at the Gathering, which confirms that he is the Keeper of Runners. Thomas is first elected as Keeper of the Runner by Minho, but becomes just a runner. Minho teaches him everything he needs to know. Later, he supports Thomas's plan to escape from the Maze by going through the griever hole.

Alby

 

"If you ain't scared, you ain't human"

- Alby-

In The Maze Runner, Alby is first seen after Thomas climbs out of the box. He tells Thomas that he is the leader of the Glade. He introduces Thomas to the Glade and the Gladers and then leaves Thomas with Chuck so he can go and take care of Ben
Alby and Minho go out into the maze to check on Minho's sightings of a dead Griever, however, Alby was stung. Minho tried to half-carry him back to the Glade. However, they arrived outside the doors just as they closed. Seeing this, Thomas slipped out just before the doors closed for the night. Minho was very angry at him for this, and said he had no hope. Minho ran off, leaving Thomas and the now unconscious Alby alone together
Thomas, refusing to leave his friend behind, grabbed Alby and started running through the Maze. He eventually realized he was too tired to keep carrying Alby, but still wouldn't leave him, so he tied himself and Alby in vines that hung from the walls and himself. As Grievers rolled up the wall toward Alby, Thomas re-directed them towards himself, and when the Grievers had picked up too much speed to miss him, he moved aside, throwing the Griever past him.
Minho then returned, and seeing Thomas dodging the Grievers gave him an idea. They left Alby tied up and ran towards the cliff together. As they approached, they braced themselves and faced the Grievers that had pursued them. They then used Thomas's dodging idea to lead the Grievers over the Cliff, saving Alby and themselves. They then started back towards the Glade, grabbing Alby on the way.
After the doors opened, Alby was immediately taken into the Homestead and given the grief serum. Soon after, he went through The Changing.
After the Changing, Alby asked to see Thomas. He started to tell Thomas how he knew everything, such as who Thomas and Teresa were, and remember what the outside world was like but in the middle he started choking himself. As Alby was about to go into more detail, he had a seizure, which was probably induced by WICKED to prevent him from revealing too much information to Thomas.
When the Grievers start coming into the Glade to pick off one kid a night Alby decides to go to the map room to "study" the maps more closely. But when the griever takes one of the boys, Winston goes to check on Alby, he has a cut across his forehead and makes it seem like he was attacked. He later confesses that he burned the maps, then he starts going off about how they can't go back because there is a disease in the world that he would rather die than get The Flare.
After Alby had undergone the Changing, it appeared as if he feared the outside world due to his memories. He became distant, and no longer trusted himself to make any decisions for the glade, leaving Newt in charge.
As the Gladers were attempting to escape the maze, they were confronted by numerous Grievers. Perhaps, afraid to return to the real world, or under the belief that the sacrifice of one Glader would stop the Grievers, Alby does the unthinkable. He sacrifices himself to the Grievers, but unfortunately, his death was for nothing.
It's unknown if Alby was immune to the The Flare or not. Alby himself seem to truly fear the outside world after the memories he got after the changing. He seem to really rather die in the maze than getting the The Flare and die which could possibly mean that he remember that he was't immune.