How does the introduction of the girl character affect the plot

A teenage girl who mysteriously arrives in the Glade the day after Thomas. She tells everyone that everything is about to change, before slipping into a coma. Thomas is sure that he knows Teresa from before, but cannot remember. Later, it is revealed that she shares a telepathic link with Thomas. They become close friends over the course of the novel and Teresa helps Thomas and Chuck deactivate the Maze during their escape. She is the only female character in almost the entire book, yet she serves little purpose except to catalyze plot and Thomas's character development.