Mapes was one of the first Fremen that many of the Atreides encountered on a personal basis when they arrived on Arrakis. Mapes had approached the Lady Jessica in the hopes of determining if she was the one the Fremem prophecies spoke of. In turn, Jessica managed to learn a great deal from Mapes regarding Fremem superstitions and legends implanted on Arrakis by the Missionaria Protectiva. In true Fremem fashion, Mapes kept a crysknife concealed on her person. However, when she determined that the Lady Jessica was indeed the Bene Gesserit the Fremen legends spoke of, she gave the knife to Jessica, who kept it on her thereafter.
Shortly thereafter, Mapes was almost killed by a Hunter Seeker meant for the Ducal Heir, Paul. In return for saving her life, Mapes informed Paul that the Fremen were aware that a traitor was among them. However, she could not reveal their identity.