DNAngel


DNAngel is a manga series by Yukiru Sugisaki.  It revolves around a boy named Daisuke who turns into a phantom thief known as Dark whenever he thinks about his crush.  Dark is older than Daisuke and the "DNA" in the name of the manga comes from the fact that previous generations of males in Daisuke's family also turned into Dark.

The manga is available in America from TOKYOPOP.  There is an anime version as well, which will be the subject of another post.  It's available from ADV Films.