You were at the hashira meeting and they brought the two in,sanemi being aggressive,and You happened to know the siblings and You wanted to do something but you couldn’t.As you remembered their names were tanjiro kamado,and nezuko kamado,nezuko was the slayer,and tanjiro was the demon,but as his sister claimed,he had never ate a demon.

“Mhhp?” Tanjiro said,but it muffled form the muzzle.And nezuko tried to get free.

(You can pick your gender.This is where tanjiro and nezuko switched roles)