Two-Face

Harvey Dent was a district attorney in Gotham City and later became Two-Face after an explosion caused by The Joker caused him to become disfigured. He was assumed to be deceased, but this was a cover to save Harvey Dent's reputation.

Background
Harvey Dent was a district attorney, but when captured by the Joker he was saved by Batman but an explosion caused him to be disfigured, which earned him the name Two-Face. Under his alias he began a killing spree on those who blamed for the death of Rachel Dawes (only when the coin landed on the scar side up). He was presumably killed when tackled by Batman, who took the blame for the killings and later went into hiding. But this was far from the truth, Harvey was housed at Arkham Asylum in order to keep intact his legacy to bring hope to the people of Gotham. Harvey's apparent 'death' led to the creation of the Dent Act, which meant no criminals would receive parole and most where sent to blackgate prison.

Events of Joker Blogs
"I've kept quiet about our little secret over at Arkham Asylum for far too long, even with losing everything." - Jeremiah Arkham hinting at Harvey's possible survival.

After Lex Luthor took control of Arkham Asylum with the help of Joker, its former owner Jeremiah Arkham lost his mind especially when his wife was left in a coma due to Joker's previous breakout. He takes over the Asylum and meets with Lex at his office, forcing him to sign back the asylum over to him. When Lex points out that none of this would hold up on court, a calm Jeremiah agrees, and says that is why he brought his attorney. He spins a coin on the table, and someone enters the room and holds a gun to Lex's head. The coin lands scarred side up, and the gunman is shown to be Two-Face, who pulls the trigger.

Trivia

 * After The Dark Knight was first released it was often rumoured that Two-Face had survived his fall and was hidden away to cover up Harvey Dent being Two-Face. However by The Dark Knight Rises it was made clear in that continuity he appeared to be dead, although its unknown if he may be killed off in The Joker Blogs at some point to fit with TDKR storyline.
 * The possibility of Dent being still alive was first hinted in the Further Evidence#2 episode Pull The Plug in which Jeremiah Arkham mentioned about keeping a secret for Gordon, which would turn out to be Dent.

Series 1

 * Staff Verification (Mentioned and photo)

Further Evidence

 * Pull The Plug (Indirectly referenced)

Series 2

 * Axes & Allies