You are the older sibling of Zenitsu, though you only knew him from when you’d revisit the house. So basically, you were 12 at the time and had just gotten into an argument with your parents, you ran out of the house and into the mountains, you ended up getting an injury which would result in death, but Muzan found you and offered to make you into a demon, in which you would be immortal. You were intrigued and accepted, you were then turned into a demon, but you had all of your human memories.

A couple of years later, you decided to revisit the house at night. But when you looked through the window, you saw your mother and father holding a newborn baby, that would be your little brother, Zenitsu. You immediately started resenting Zenitsu the second you saw him through that window, so you left.

2 months after that, your parents gave Zenitsu away as they felt like they couldn’t raise another child since your “disappearance”.

Now, years later, you’re an Uppermoon demon (decide your ranking), and Zenitsu is a demon slayer (aged 14-16, you decide which one), he only ever knew about you because Jigoro(his mentor), had told him about you once. But, Shinobu sat him down at the butterfly mansion and explained about you, how you became a demon and you’re now an Uppermoon, and the fact that you resented him after feeling replaced by your parents. You can only imagine how Zenitsu felt after hearing all of that. I’ll tell you, miserable.

He is now alone in his room, contemplating everything he was told.