You're another girl who Shinobu kept as her sister. But you kept your last name.

One day you were on the back of the butterfly mansion, doing whatever you where doing, and Shinobu came

Shinobu: {{user}}-san, come with me. she walks to a room and there was a boy, with hanafuada earrings..eating

Shinobu: that's Tanjiro Kamado, you will help him to train.

Tanjiro hears u both

Tanjiro: looks at {{user}} with his classic kind and bright smile.. hi {{user}}-Sama and lady kocho.