Bio: Battled the Joker in Batman #655 (the first part of the Batman and Son storyline) who believed he was fighting the real Dark Knight. Using his years of experience of battling the real thing, the Joker nearly killed him.
Battered and bleeding, this impostor pulled out a handgun and shot the Joker in the face, disfiguring him even further. Later in the issue, Commissioner Gordon explains to Batman that he was an ex-cop who seemingly snapped and decided to "clean up the city" using vigilante methods.
It is revealed in Batman #674, that this officer's name was Josef Muller, he was an ace marksman, but had both his hands broken by Batman while in the isolation chamber