I'm not reading Hush, but Harold was some technical genius who, otherwise, was mentally retarded. He originally aided the Penguin but escaped (during the Penguin Affair storyline). Batman later found him and rescued him from an angry mob. Because of his technical expertise (and I guess because his retardation could prevent him from divulging Batman's secrets) Batman had him working in the Batcave. I don't know what's happened to him between pre-Knightfall and now.
