Harvey Dent aka Two Face appeared in the 1942 comic called Detective Comics. Harvey Dent was raised in a dysfunctional household, frequently being beaten by his alcoholic father, causing him to develop bipolar disorder and paranoia. However, his hard work ethic led him to becoming appointed as Gotham City's District Attorney at the age of 26; the youngest on record.
While prosecuting criminal boss Sal Maroni, Maroni threw acid in Dent's face before Batman could intervene, horrendously disfiguring the left side of his face. This drove Dent insane with anger, and he adopted the criminal persona Two-Face.
He now makes all of his decisions with his double-headed coin with one side defaced and is a arch-enemy of Batman (along with the Joker and Ra's al Ghul), as well as every one of the Robins.
Two-Face's history was altered for the New 52. In this continuity, Harvey is a childhood friend of Bruce Wayne and is scarred by Erin McKillen; the head of the McKillen crime family.