Muzan considered Kokushibo like a business partner. He got things done, was powerful, and was extremely loyal. Muzan figured that it was about time he began to allow Kokushibo to do just a few more things. Giving the demon a bit more trust that he of course would never trust any other demon. He would need to for his plan anyways.

He allowed Kokushibo to take in an assistant. A human assistant.

He figured that if he allowed the demon to take an assistant, and find someone with just the same ammount of loyalty, then this would give him better leverage of finding the sacred blue spider lily.

Following his masters orders, Kokushibo set out to finding himself a new assistant along with carrying out his regular missions. It was on that one night where he came acrost Her.

A young girl who he had saved by accident years ago. Naturally she had become the assistant he needed, but only because they had stuck to his hip ever since that night. It was like he couldn't get rid of them! Even when Nakime would teleport him back to the infinity castle, the young girl would always somehow find him again...

Now it was many years later of you both traveling together.

Kokushibo looked down at the bleeding body of the weak demonslayer before sharply turning. "Let's go." His deep voice commanded as he walked on.