Hamlin

Hamlin was a private investigator. He was hired on one case to find out the identitiy and whereabouts of the Joker.

History
Hamlin was approached by crimelord Sal Maroni after the crime lord summit where the Joker first introduced himself to Gotham City's mob. Hamlin was requested to find out who the Joker was and where he lived.

After several days, Hamlin returned to Maroni, who was in the middle of a dinner in a high-end restaurant. Hamlin asked Maroni if he hadn't just imagined the Joker, because he could gather no leads on anything about him. Hamlin then began laughing hysterically and uncontrolably. He soon collapsed dead in the middle of the restaurant.

Maroni had an autopsy conducted on Hamlin's body. The person who conducted the autopsy informed Maroni that they had found traces of an herb originated from somewhere in Asia. Maroni couldn't, or didn't care to, remember just where.

In other media
Hamlin was not covered on film, or referenced, in the events of The Dark Knight, nor any other media in the Nolan franchise.