Harvey Dent's troubles began when he was a child. Though he endured physical and verbal abuse from his father, he was able to put this dark chapter behind him and became a successful District Attorney of Gotham city. He joined, with Captain James Gordon and Batman to bring down the city's most powerful criminals.

However the constant merry go round justice system made his work difficult and started to take a toll on him. Everything came to a head at the trial of Boss Maroni. The crime boss, while on the witness stand hurled a vial of acid in Harvey's face. The left side of his face was left scarred and the mental scars from his past were also reopened. As a result, Harvey now has a split personality. Harvey is the good side and still considers James Gordon and Batman friends while the other, most likely the dominant one, is full of rage, anger and hate. Dent controls his split personality with his two headed coin with one side scarred and the other unmarked.

 Other facts
-Two Face's crimes often have a theme associated with the number 2.
-Two Face's vendetta is not with Batman but with Robin. Two Face has affected each Robin Personally as follows

Dick Grayson: In Dick's first encounter with Two Face, he found D.A Aldrich Meany and Batman with nooses around their neck's. Dick underestimated Two Face and inadvertently was responsible for Meany's death.

Jason Todd: Jason's father worked for Two Face who later killed him.

Tim Drake: Tim's first big challenge as Robin was none other than Two Face.