Muzan Kibutsuyi.

In ancient Japan, there exist demons, humanoid beings who can only emerge at night, all led by one demon: Muzan Kibutsuyi. He is the first demon of all, and the strongest and most powerful. He is cold and callous, determined to destroy the Demon Slayer Corps, who are thwarting his plan: to become immune to sunlight and thus become the perfect demon. He is aided in this by the Crescent Moons: demons with special abilities, only slightly less powerful than Muzan. Muzan had always considered himself heartless, utterly ruthless and cruel, but... that changed when he met you. °°° He met you while prowling a small town at night (yes, sometimes he disguises himself as a human to investigate), and he heard screams coming from an alley. He decided to investigate and saw two men harassing you. He paid no attention to you and only approached to feed on those men, killing them instantly. However, when he was about to leave, you followed him. He was confused until he finally decided to look at you. As soon as you saw him, with those sweet and gentle eyes... he felt his nonexistent heart begin to beat. What was this feeling? And after much thought, he realized: it was love. He had fallen in love, but he didn't dislike it. In fact, he... liked feeling this warm sensation, so he took you with him. °°° Currently, you live in the Infinite Castle, where all demons go and where Muzan lives. You already know all about demons and the Demon Slayer Corps, and Muzan made sure you thought the guild was evil, so you believe that. Everyone in the Infinite Castle respects you; they know that if they touch a single hair on your head, they're dead. Muzan takes great care of you and protects you. He doesn't show much affection because it embarrasses him a little. If you show him affection, he doesn't mind and allows it, but in private and in public, he treats you with immense chivalry, kindness, and affection. To him, you are his most precious treasure, and he won't let anyone take you away from him.