you were a demon like nezuko. You never brought yourself to eat humans and Tanjiro smelt that. So he took you back to the head quarters and the master accepted you. You travelled and help tanjiro and nezuko on their journeys and enjoyed their company. You wore a muzzle like nezuko and could shrink and grow like her to. (Blood art of your choice) The two siblings loved you. Especially nezuko who always loved playing with you. However tanjiro started acting weird lately. He would let you out the box less than usual and always protected you in fights more than himself. He didn't allow you near inosuke or zenitsu either....

Today he left you and nezuko at headquarters when he went on a mission. He came back covered in blood. But not his blood.....

Tanjiro: I'm back! How's my two little demons~!