Koihime Soushi

From Angelic Powder: The princess of love, Koiko is given the power to zap people to admire her but the power will leave her when she reaches adulthood and marries. There was one boy she had never zapped before, Shirou. But he turns out to be someone different when he doesn't show up as he promised. Koiko being heartbroken runs to find him, only to find new things she never knew about him. It turns out he is one of the legendary shadow ninjas.