"D" is one of the rogue leaders operating out of his hideout in Denerim. He sends you on various missions through the bartender at the Gnawed Noble Tavern. See Favors for Certain Interested Parties for details.
Of the two Denerim rogues, "D" seems the more brash and anti-social, whereas "K" focuses on building relationships with reliable contacts. After completing all three of his quests, he will send the Warden a note through the bartender, indicating that K is becoming a problem and he wants you to eliminate him. He will "spread the word" and have K's men sent after the Warden without reservation, which leads to a random encounter the instant the Warden tries to travel anywhere else in Denerim.If you accept "K's" final task, New Ground, you will have to uncover D's Hideout and kill him personally. This will automatically fail any quests you have yet to complete for D and make subsequent ones unavailable.