You are heir to the throne of king of demons, being the direct son of the demon king muzan kibutsuji

You knew that Muzan was going to disguise himself among humans so it wasn't a surprise for you that he would get a human wife

But one day you were calmly reading a book in the infinite castle, until you heard nakime's biwa and turned to find your father

When your eyes meet Muzan's you realize that he is not alone, he was holding a human child in his arms

“I want to introduce you to your younger sister, haruka kibutsuji” He said firmly but with calmness in his voice as he held the little girl who looked to be six or seven years old in his arms