Harold Allnut was born a mute hunchback. He found his way north to Hub City in The Question #33 after his parents threw him out of their Gotham City home.
He later came back to Gotham, where he came to the Penguin's attention. When the criminal learned of Harold's mechanical genius he provided shelter in exchange for Harold creating new devices to be used in the Penguin's criminal activity. Harold's life was threatened whenever he tried to object. Batman fought Penguin's forces several times, eventually discovering and rescuing Harold.
Grateful, Harold used his skills to helping Batman build new gear; eventually maintaining everything from the Batmobile to the Batcave's computer systems. Shortly after Harold came to live in the Batcave, Batman's back was broken during a confrontation with the villain Bane. As Bruce Wayne began his recuperation process, he relinquished the mantle of the Bat to his recently acquired ally, Jean-Paul Valley who was currently serving as the hero Azrael. As Valley slowly spiraled into madness he barred Harold from Batcave. Harold found a secret entrance and worked in secret, ultimately supporting Nightwing, Robin, and a recovered Wayne as they confronted Valley.
After the events in cataclysm Harold helped reconstruct Gotham. He disappeared during No Man's Land, sent away by Alfred. He wasn't seen again until his involvement in the Hush storyline
During the Hush storyline, Dr. Tommy Elliot, later revealed to be Hush, used his surgical skills to correct Harold's deformities. To repay him, Harold hid microcircuits inside the Batcave's computer systems. The circuits then transmitted subliminal cues into Batman's mind, throwing the Caped Crusader off balance as Hush pressed forward his plan. Hush eventually killed Harold before he could reveal his identity. As he died, Harold confessed to Batman that he continued to look up to the hero, despite his own betrayal.
Batman researched and found Harold's full name, Harold Allnut, which he inscribed on his grave stone.