When she is first seen in The Scorch Trials, it is implied that she and Sonya had had some kind of authority over Group B before Teresa arrived, possibly as a parallel to Alby, being similar in appearance and in a stronger, no-nonsense personality than her counterpart Sonya. She and Sonya help Thomas feel betrayed by Teresa by dragging him through the Scorch in a burlap sack. Later, they feel guilty and save Thomas from being killed by Group B by talking Teresa out of killing him.
Harriet follows Teresa's lead and allows WICKED to give her memories back. She escapes WICKED shortly afterward with the others of Group B, plus Teresa and Frypan, and goes to Denver. She and the other Immunes are captured by the Right Arm as part of a ploy to take down WICKED. Later, as WICKED's headquarters are being blown up by a string of bombs set up by the Right Arm, Harriet is one of the many Immunes who make it through the Flat Trans to safety.
In the book, she is pretty, and described with having dark brown skin and hair cut almost to her scalp.
In the film adaptation, Harriet has warm brown skin and is depicted with long dreadlocks that fall a little past her shoulders.