Whistler

 was a junkie heroin addict that Charlie has turned as a confidential informant for the FBI. He died from a heroin overdose given to him by Quinn.

Charlie
Charlie first met Whistler as part of an undercover operation. She turn him into a semi-reliable source of information for the FBI. She came to care for him and his future, even going so far as finding him a house he could buy with payments he received from the FBI.