DN Angel by Yukiru Sugisaki

This is volume 1 of a manga series and I understand why it's a hit. Daisuke is an 8th grader who gets a surprise when he turns 14. He discovers that his mom and grandpa have been training him to take over the family business (art theft!). Whenever he comes into contact with the girl he loves, he changes into a studly man named Dark who steals art. But then he changes back to Daisuke. So the bulk of the novel is about the first few thefts and Daisuke trying to learn how to control his change.