Two-Face as he appears in DC Comics
Harvey Dent - also known as Two-Face - is a violent, insane gangster and one of Batman's regular foes. Formerly Gotham City's district attorney, he had vowed to rid the streets of mob crime, becoming one of Batman's allies early in the Caped Crusader's career. This would not last, however, as he was attacked by a powerful mob boss who threw acid in his face, leaving his face hideously scarred on one side.
As a boy, Dent grew up in a harsh domestic environment. He would repress all of his anger and hatred, leading to him developing schizophrenia and bipolar disorder later in life. Though he received treatment for his condition, his injury would force those disorders to surface again, manifesting as a new identity. And thus, Two-Face was born.
As Two-Face, Harvey became obsessed with duality and based many of his crimes around the number 2 and double themes. He also came to believe that chance was the only thing that mattered in life and that became the only rule he lived by. Two-Face carries a double-headed silver dollar and flips the coin every time he makes a decision. One side of the coin was damaged when his face was disfigured and so he usually makes violent decisions when the "bad head" comes up. Batman has tried for years to help Two-Face recover from his mania and return to the man he was, but Two-Face refuses to be suppressed and will never allow Harvey Dent to take control again.