DN Angel
Warning: Wikipedia contains spoilers.
D.N. Angel is a manga by Yukiru Sugisaki, which also spawns an anime with the same name. Not to be confused with DNA, which belongs to a totally different anime genre. The anime spans a total of 26 episodes, and, although it tries to pass off as a shounen, it is more of a shoujo anime (angels, bishounens, much romanticism, et cetera).
Background story
For countless generations, the boys in the Niwa family has served as "vessels" in which the spirit of Dark, the Phantom Thief, used to manifest himself. Every boy, at the age of 14, will turn into Dark whenever he feels a deep longing for a person that he loves most (i.e, his girlfriend).
D.N.Angel revolves around the adventures of Niwa, who has to juggle his responsibilities as a student, acting as the current alter-ego for Dark, and being a good son, and at the same time evading the pursue of the authorities (Dark, although a good guy, is a thief) and Dark's arch-nemesis, Krad.