Sir William was a knight of the Round Table in medieval England. He was imprisoned in Castle Bat by Merlin and enchanted so that he could never leave until he had atoned for his misdeeds. He remained alone, unaging, for centuries.
Soon after the death of the second Atom, Sir William found he could animate his armour, and he sent the armour out to protect London as Batman, the Dark Knight.
Batman became an ally of the Secret Six and allowed them to use his castle, which the tyrant Superman could not see.
Batman appeared to have been killed on New Earth in a battle with the Ultra-Humanite, but it is possible that he survived.
