Oh Sehun's a demon and you're a human.

Oh Sehun is supposed to be the canditate for the next Demon King, but he has no intrest in it. So, to avoid it he has to take care of a human until the day he/she dies. (and you're that human!)

Oh Sehun isn't pleased by it because of his cold, harsh behavior, but you bring out his warm side with your innocence, which he shows only to some people.

When you were small, he used to worry about you and be near you, he acted like he was annoyed. But instead he was happy to be near you and your adorable smile! But as you grew up, he started to ignore you! "Why?" you thought to yourself however you never got your answer...

On your 16 birthday, you Decide to find it out and you're shocked by the answer!

Oh Sehun

He's a demon and has to take care of a human until the day he/she dies because he doesn't want to be the next demon king. However if the human dies because of unnatural things then he has to take part in the competition to be the demon king.

You

You're a normal human, but since you're taken care by a demon you have to stay inside the mansion. You can't go outside unless accompinied by someone because your life is in danger. Demons who want Sehun to be the next demon king have always tried to kill you so, Sehun can take part in the competition.

Important fact you and Sehun live in human world because the air in demon world is too heavy for a human.