Rachel was a member of Group B. She is likely named after Rachel in the Bible, as it is suggested that the girls in Group B are named after religious figures. She is first mentioned by Aris in The Scorch Trials, who explains that she was the only one who cared for him, and they became friends shortly after Aris arrived in the Maze. She could communicate telepathically with Aris like Thomas could with Teresa. From what Aris says, Rachel is Group B's equivalent of Thomas.
However, like with the Gladers, after Group B escaped from the Maze, Beth, who seems to be Gally's equivalent, stabbed Rachel while she was under WICKED's control. No one jumped in the way like Chuck did for Thomas, leading Thomas to believe that he should have been killed. These fears are unfounded, of course, for it is later revealed that Chuck had been under WICKED's control when the knife was thrown and he jumped in the way.