Why do you think there was only one girl in the story and why was she the last to arrive?

Although this topic is not explicitly discussed in the book, Teresa's appearance in the Glade is a catalyst for change. Everything about her is different: She is the first female to come to the Glade, she arrives unannounced and off-schedule, and she is comatose when she arrives after delivering a cryptic but frightening message. She upsets the natural order of the Glade. Her introduction was meant to trigger the Ending, something the Creators explicitly planned. As part of the larger experimental nature of the Glade, it may have also been to see how the Glader population would react to her.

The male-only makeup of the Glade resembles a prison. In prisons men and women are generally kept separate to ensure order for the same reason. This also prevents any type of sexual contact between the men and women and ensures there are no unplanned pregnancies or assaults. However, Teresa's femininity is not a major plot point in the novel. Although Dashner makes reference to the potential dangers when some of the boys call "first dibs" on Teresa, her safety is not a major issue. This may have been a decision by Dashner to avoid the topic in a young adult novel.