Who is Mallory from The Field Guide and what is their importance?

Mallory Grace is Jared's older sister. She is thirteen-years-old, loves fencing, and isn't at all afraid to fight. It is Mallory, in fact, who starts the fight with the creature when she knocks a hole in the kitchen wall, an act that takes out the creature's nest. As a result, she is the first victim of the boggart's pranks. Initially, Mallory struggles with the thought of listening to Jared because he is younger. Without her, Jared would never have accomplished his goal.