Headmistress Queller is the headmistress of Constance Billard. She first appears as a strict disciplinarian determined to find whose is responsible for breaking into school grounds during School Lies.
She seems to have the most interactions with Blair Waldorf, easily the most studious of the characters and most concerned with her academic career. She attempts to help Blair get into Yale after her incidents with Rachel Carr and listens to her when Blair pleads for Chuck on his behalf when he's out of control. She reveals that she, among other faculty, is fully aware of the Gossip Girl blog.