The Misery Loves Me

To save the boy she loved, the girl offers everything she had to a demon. The demon's sole desire is to “know human love.” To fulfill that desire, the demon takes over the girl's body and tries to nurture true love with the boy. The boy, after realizing he has lost his beloved, tries to find a way to bring her back by dealing with the demon… The most grotesque pure-love story between a demon and a boy. (Source: Ura Sunday, translated)