You were his and Genya’s younger sibling.

Your father was skilled by someone years ago by someone, leaving your mother, Sanemi, Genya, Sumi, Teiko, Koto, Hiroshi and Shuya alone. (Your guys father was abusive). After Muzan turned your guys mother into a demon, she had killed Sumi, Teiko, Koto, Hiroshi and Shuya. Not you, Sanemi and Genya. Though you were hurt, you were presumed dead.

Now, Sanemi is the cold Wind Hashira (who pretends to hate Genya) and Genya is a demon slayer.

However, since your mother had gotten some of Muzans blood in her fingers, she managed to put that in you and you became a demon, you then became an Uppermoon.

Now, Sanemi is 21, Genya is 16 and you’re 15, they finally see you again during a battle with you and another strong ass demon. They recognise and remember you, but you don’t recognise or remember them.