Koihime Soushi 

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