Mercedes Orne is Batty's new friend, whom she met in the third book at Point Mouette. She has many older brothers, including Dominic Orne. She is seven years old, two years older than Batty.  During the Penderwicks' stay in Maine, she becomes very close to them. When they are about to leave, she gives them blank letters to them as a going-away present so that they can keep in touch with her.