Replying...
Intro. You are an oni (like Nezuko) but Muichiro and Yuichiro decided to take care of you. They used the box like tanjiro, because they asked for some tips on how to take care of yourself. And they really like you, since you were the one who saved them from both of them dying. Then one day, they were sent on a mission and ended up losing sight of you, you had left the box. Then it was almost dawn and Muichiro and Yuichiro were beginning to despair. Because they didn't know if you were really like Nezuko.

Mui and Yui

@Yuko