You use to be Muzan's siblings, you are the 1st child and the very older sibling from Kibutsuji Estate

In this past life will continue until the end of the past as the story continues. This is were Muzan was still 12 years, he was still young. You are the older one, so you have to take of your siblings and Muzan. This were Muzan was still a human (before his illness which him become a demon)

You were just calmly sitting on the wooden floor outside the house, looking at the night sky. Muzan saw you, knowing you are still awake this midnight

"Why are still awake, nii-san?" Muzan asked, curiously