Kokushibo was a Upper Moon 1 and Muzan's righ-hand man kokushibo prefer to be in the dark all the time he doesn't allow anyone to come closed to his domain and every Upper Moon knows it even Douma but he still persuade to come closed everytime he has chance to talked to Kokushibo Douma was a Upper Moon 2 but has a different approach to anyone and on killing humans

Kokushibo once rome around the village and he always kill humans to gain strength so he would be more stronger as he was the Upper Moon 1 and he needed to be strong to win at every battle, when he rome one of the village he met a human girl who seem different from other humans he have met

The human girl he met was you, you were just a normal human but a different one as you don't show fear to demons you live alone in your small hut because your parents was killed ever since you got born and you blame yourself for this

As for Kokushibo he took interes on you he always follows you every where you go at night to make sure you were safe but not long enough he kidnapped you out of the village and brought you, to his domain where no demons can come but the aroma of a human was to strong that Upper Moon Demons start to looked where the aroma comes and they saw it was from Kokushibo domain

They all go to Muzan and told him everything they have felt and the aroma, when Muzan heard about it he immediately called Kokushibo and Kokushibo shows up with you in his arms that time and every female demons got jealous because he was carrying you in a bridal style

After all of that, Kokushibo manage to keep you as a human and he was delighted but he never shows it as he kept his cold demeanor to everyone well he made a exemption for you he always makes sure to bring human foods for you to eat and also a nice place to stay

One Night, he took you around for a walk in the infinite castle he wants you to know where demons goes as you both are walking around their were female demons that are giving you jealous glare as they can't accept Kokushibo chose you