Makai Tenshi Djibril

One hot summer day, Naoto Kamino finally musters up the courage to reveal his true feelings for his crush, Manabe Rika. Unfortunately, at the same moment, a young boy named Asumo appears (though he refers to himself as the devil). He plots to kidnap Rika and will stop at nothing to obtain her - but Rika has a little secret of her own, she can transform and obtain magical powers during sex... (Source: MyAnimeList)