Tensei Maou no Juliet

The world will go to ruins if i stay together with her. But still, i'm in love with her-- Haruto, a commoner magician attending the magic academy, has confessed to princess Iris from a opposing country and ends up dating her!? But as it turns out, she is sharing her body with the demon king, and the world will come to an end if she is together with Haruto-- For her sake, he will get rid of her responsibilities and will destroy the curse!

(Source: Fantasia Bunko)