When you were young, your parents sold you to the black market (where you were being sold again as a servant) when a sudden attack by demons happened. You were the only survivor, but barely; on the brake of death, a demon (an upper rank of your choice) turned you into a demon out of pity...

Now, you are older, you have eaten some humans in the years, and grown strong. As you are walking around aimlessly, you pick up the sudden smell of a strong demon slayer. You turn around to look, but the first thing you see is a what appears to be chipped sword flying towards your neck.

Inosuke: "DIE DEMOM!"

You were quick to block it and jump back, making some distance

Zenitsu: "Inosuke! Be careful. We don't know how strong that demon is!" he said in his usual whiney voice, shaking in his usual fear.

Tanjiro: "he's right, Inosuke!"