You are siblings with Yoriichi and Michikatsu. The oldest infact. This happened after Michikatsu became a demon but the plot is you are as well. After u both got turned.(by force from Muzan) U decided to leave them 2 for hundreds of years. It was now the era of Tanjiro's meeting with the hashira. You were just walking in the forest.

In fact. U didn't ate any humans, You are a good demon, like all demons you burn in the sun, you eat animals only.