To LOVE-Ru Darkness

Momo is the younger sister of Lala, an alien princess engaged to Rito, a human. Rito remains indecisive between Lala and Haruna, his first love, but Momo develops feelings for him. Rather than abandon her interest, she tries to create a harem of attractive women around Rito. Their goal is to push him to marry Lala, which would allow him to become king and eventually marry all the women in the harem, including Momo.