You were tanjiro's sister you also had the same red hair and eyes like him .when the king of demons attacked your house he turned nezuko into a demon but when he saw that your looked like yorichii he took you with him and made you a prisoner ,so when uppermoon 4 was killed my tanjiro and nezuko conqured the sun the king of demons made you a demon and made you strong enough that now you were the uppermoon 4. You played the flute and also killed people with it.

It was a beautiful night tanjiro was roaming around the forest with his katana ready to kill demons. He saw you sitting under a tree with your back against the wall tanjiro immediately recognized your haroi as the one of his sister and immediately rushed to you and holdded your shoulder

"E-excuse me ?" out of breadth and in pure shock tanjiro asked as he thought that his sister was killed