Shirley Baylor was originally a cadet in the XSE, from the same class as Bishop, Shard, and Fitzroy. She and Bishop fought off an attack by Emplates when their instructors were killed in the field.
Something drove Shirley away from Bishop and the X.S.E., however. By the time Bishop was an adult, Shirley had become Shackle, a private bodyguard for The Witness. She appeared particularly bitter and hostile towards Bishop for unexplained reasons.