Kurokishi-san ni Soudan da!

Black Knight, the strongest knight in the Demon King's army, fell in love with Alicia - a human woman. But Alicia is already engaged to become the Demon King's wife out of a marriage of convenience! Although the Demon King is scary, he's a naive virgin. So Black Knight tries every possible means to decieve the simple-minded Demon King, all while Alicia is trying every possible means to make her marriage of convenience a success. Egos, maliciousness, and humor collide in this comedic, other-worldly battle to steal one's lover! Source: Shogakukan