DN Angel

Daisuke Niwa is an ordinary, somewhat unlucky middle school student until his fourteenth birthday, when a condition passed down through the men in his family for three centuries begins to affect him. Whenever he sees his crush, Risa, he transforms into Dark Mousy, a phantom thief. Dark Mousy’s feelings for Risa’s twin sister, Riku, make the transformations more complicated: when he sees her, he turns back into Daisuke. The only cure for the condition is for Daisuke to get Risa to fall in love with him.