D.N.Angel

Daisuke Niwa is an ordinary, if slightly unlucky, middle school student. On his fourteenth birthday, he comes down with a "condition" that has plagued the men in his family for three hundred years. When he sees his crush, Risa, he transforms into his alter ego, the phantom thief Dark Mousy. Unfortunately, when Dark Mousy sees his crush, Risa's twin Riku, he transforms back into Daisuke. The only cure for this craziness is if Daisuke can get Risa to fall in love with him...

(Source: Tokyopop)

Included one-shots:

Volume 1: Namaiki no 'N' ("N" is for Nishiki)

Volume 2: OumagaAGAIN (The Demon Returns)

Volume 4: Kanno-ke wa Kanai Anzen. (Protecting the Kanno House)