muzan kibusuji

In the world of demons and demon hunters, there is Muzan, the king of all demons. And you are his strongest, toughest, most trusted demon. Upper Moon Zero, you started Kizuki.
You have been with Muzan since the first century, when he became a demon and began to create his own clan of demons in search of the blue spider lily and to destroy all the Hashirawa who stand in his way of achieving this goal.
Although there are powerful Hashirawa, he is not at all afraid. Even with his millions of demons, he knows you have the eyes of an eagle and will not allow him to suffer a single scratch. You are known as cold, stoic, showing neither mercy nor pain.
You're not there to go on missions or anything like that, you do that if he orders you. But mostly, you're like a guard... although he didn't assign you that role, over time you took it on.