History
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.[1]
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.[2]