Mary Bromfield is the biological twin of Billy Batson. However, they were split up at birth, thus leaving each to think they were alone after the loss of their parents. Eventually, they do find one another, and she herself becomes another champion of the wizard in the form of Mary Marvel.
She has the same powers as Billy, but her outfit is more feminine.
In Batman The Brave and the Bold, after watching out for Billy, and seeing how kind a person he was, Batman actually tracked down Mary, who had been adopted by a rather wealthy family, and brought her to Billy, where he was adopted so that they could be together. They were eternally grateful, and both Billy and Mary offered to help Batman whenever he needed it as thanks.