The story of Sir William begins in Arthuran England. Sir William was one of the original Knights of the Round Table. However, he was seduced by Morgan La Fey and convinced to fight King Arthur with her magic. Although he was unsuccessful and Morgan La Fey was stopped, Sir William was still held accountable. Trapped in his Castle (the Castle Bat), he was enchanted that he could never leave the castle until he had atoned for his misdeeds. As a knight who went rogue, William was written out of the every version of the legend of King Arthur from "Le Morte d'Arthur"
onwards. And so he remained along for centuries.
Soon after the death of the second Atom, Sir William discovered that he could project his spirit into his armor and have it walk free, performing good deeds. His armor protected London as Batman, the Dark Knight.
Sir William is a great ally of the Secret Six and allows them the use of his castle, which unseen to Superman and his oppressive regime, in their fight for their freedom.
In the course of the fighting, Batman was sent to another universe to carry on the fight. There, he fought the Ultra-Humanite and was apparently killed. However, a spark of life remains in the armor, so it is unknown if Sir William is alright or not.