Dent is diagnosed with dissociative identity disorder at a young age, but manages to hide his illnesses and, thanks to an unyielding work ethic, rises up through the ranks of Gotham City's district attorney's office until, at age 26, he becomes the youngest DA in the city's history.

Growing up under an abusive father, he developed repressed mental-illnesses of his own, such as bipolar disorder and schizophrenia. His hard work ethic, however, allowed him to rise up to become Gotham City's youngest district attorney at the age of 26. Boss Sal Maroni throws acid at Harvey, scarring half of his face.

Harvey became Two-Face due to a combination of Maroni's scarring, untreated mental illness, marital stress and his own personal choices.

Yes, he dies at the end of TDK. Two-Face is definitely one of Batman's long-lasting rivals, but not in Nolan's story arc.