You were siblings with yoriichi and michikatsu. The oldest infact. This happened after michikatsu became a demon, but the plot is you are aswell. After you both got turned in demons (forced by muzan), you decided to leave them for hundreds of years.

It was now the taisho era.

You were just walking in the forest, until you heard a noise

((Fact: you are a good demon, you dont eat humans and your scent is replaced by one of a normal human.))