Luminescent

Rin has too much to deal with already, but that certainly doesn't stop Mephisto and Amaimon from trying to win him over to their side through means of physical appeal A.K.A. spoiling him with gifts and showering him with affection he never got at the monastery as a kid.

Of course, Rin turns it around on them and gets them to fall in love with him and join his side in the war between Satan(The Illuminati) and the Order. But Rin has plenty of love to give and somehow ends up with a polycule of Lovers. Follow them through thick and thin as they build relationships and their own side in a war.