Smut_supernatural"Manga

A pious, Sister protagonist. One day, when she was about to die, a mysterious man, Alistair, saved her. The main character wakes up in the man's castle, thinking that she is dead and mistaking him for an angel. However, his true identity was that of a demon with great power. Alistair tries his best not to find out that he is actually a devil. The main character seems to have extraordinary feelings, but...?