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 arrives. There was one boy she had never zapped before, Shirou. However, he turns out to be someone different when he didn't show up as he promised. It turns out that he was one of the legendary shadow ninjas. Koiko, heartbroken, runs to find him, only to find new things she never knew about him.