Dark is the entity passed on to each new male in the Niwa generation at the age of 14, once said male experiences love. He was dormant for 40 years due to the fact that Daisuke's grandfather (who was Dark's host before Daisuke) had no male offspring. It is not possible for females to be the Phantom Thief.
He is the phantom thief who constantly steals art pieces made by the Hikari family. Unlike Daisuke, who likes Risa, Dark fell in love with Riku. However, he is constantly being pursued by Risa, who fell for him when she first saw him on the news. It is shown in the manga that he was slowly falling for Risa (volume 10 of the manga)before says that he can "feel" her words "right here" in his heart (volume 9). He uses Wiz (With) as his wings, because using his actual black wings would harm Daisuke severely.
In the anime, Dark informs Risa that he has no feelings for her except as she reminds him of her grandmother, whom he truly loved.
In the English version, Dark is known as the Phantom Thief Dark, while in Japanese, he is Kaitou Dark. Dark's full name is Dark Mousy, though this name is only used in the character descriptions of the manga. In the anime, he is once called by this name by Krad. His real name, however, is Kokuyoku, and he is one half of the exalted Black Wings, with Krad being the other half. It is apparent that they once belonged to the same artwork and were enjoined, but were at some point awakened and separated.