Tenshi no Kagai Jugyou and Purimu no Nikki are both works of Nico Pun Nise. Tenshi was kind of rushed in the end while Purimu has a happy ending. In Tenshi, the MC seems to be refered to as the younger brother of the Succubus King. He manages to absorb the powers of his Mate and applifies the purity of the magic(Much like Yuu). In Purimu, Yuu becomes king of succubi and father to a new born succubus(which may or may not be the succubus girl in Tenshi).
Is it possible Yuu and Kota are siblings? And if so, is Yuu evil now?