Harvey Kent a.k.a Harvey Dent was a friend of Batman. But one day he accused Sal Maroni, a crime lord, for murder. It was proved that he did it when Maroni's "lucky" two headed coin was found on the crime scene with Maroni's fingerprint on it. In rage Maroni threw Sulfuric acid at the left half of Harvey's face, scaring it. His horrible look caused him to go insane and scar one face on his coin. He flipped it to chose between a life of crime or good doing. Sadly, a life of crime won.
So he decided to decide everything on the flip of a coin. Thus one of the strangest and unpredicable ciminals, now calling himslef Two-Face was born.