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.