A.K.A.: Harvey Kent, Apollo, Janus, Mr. Duall
First Appearance: Detective Comics I#66 (August 1942)
Abilities: Extensive knowledge of criminal law, hand to hand combat
Weapons: .22 pistol, two pistols at once, double-barrel shotgun
Quotes: "Two sides...handsome, like mine once were! Now one is scarred, ugly like mine!"
Worst Act: murder, stealing anything two related
Created by: Bill Finger (writer) and Bob Kane (artist-credited), Jerry Robinson/George Roussos(inks)
Breaking with the order somewhat, but given the date I felt this was the most appropriate choice.
Harvey Dent (Kent in the first few stories) was the dashing and crusading district attorney of Gotham City when "Boss" Moroni threw a bottle of acid into his face during his hearing. Batman deflected the bottle, making strike only one side of Dent's face, but the damage was done...
The early stories were the better ones I think. Two-Face was a character split right down the middle. With this two-headed coin, he could rob a jewelry store first then pay off a mortgage the next. The character had a proper end, with plastic surgery (and presumably a good lawyer) giving him a happy ending.
Two-Face would be brought back a few times, first with imposters, then Harvey himself getting scarred again (this time thanks to dynamite). The 60's would be coin free, although there were some plans to bring Two-Face onto the 1966 series.
Two-Face would make his big return in the 1970's and would stay more or less at the same level to the present day.
Personally I always enjoyed the idea of Two-Face more than the actual stories. A character that could be equally good or evil but with the flip of coin should be a pretty good story engine, yet with the start of the 80's the writers seem to turn Dent into just another thug, most of the time just flipping the coin and stealing things dealing with 2.
Frank Miller did introduce the concept that Dent, Gordon, and Batman were sort of the first hero team in Gotham, although Dent's role in Batman: Year One was shamefully short.