A Demon Hahsira? (Kny x reader)

Prologue

There was this strange kid who was born very different from normal ones. One day something tragic happened to the little h/c child, and it all came down to being trained by Yoriichi Tsugikuni himself to be a demon slayer. What would happen next if the Y/n was turned into a demon, and how did this story all happen? Well then, let's start at the very beginning...

~Flashback~

A woman in her early thirties was walking down the hallway going back to her room after getting a cup of water, but then she stopped in her tracks when she suddenly felt pain in her stomach. The thing about that woman was that she was 9 months pregnant with twins.

The woman called for her husband, and he rushed his wife to the hospital. The woman was now in labor and was now giving birth to their soon-to-be-born children. All you could hear was screaming, and the people helping the lady give birth. (To be honest, I have no clue what giving birth is like, and I don't want to know)

Suddenly silence occurred, soon after the room was filled with crying. Two babies were held in front of their parents. The parents were filled with joy, but one of the babies caught their eye... One had a weird mark on their chest that went up to their neck and their cheek, their eyes were different from the other and had little fangs poking out of their gums. This upset the father, but the mother was just concerned. The nurses told them that they would take some tests from the child, and give info as soon as possible.

They give the other child to the parents before taking the other one away. After an hour later the nurses came back with the baby and told them that there seems to be nothing wrong with the child. They presumed that the baby was just born like this. The nurses had the specialist test the child and found nothing wrong with it, but they would try to find a conclusion on the strange appearance.

After a week the parents with their two children got to return home, but the father didn't accept their child with the weird markings, different colored eyes, and fangs. The Mother found it upsetting seeing that her husband had hatred towards one of their children just because of their appearance.

Five years passed and the two children have grown. One of the children had fine skin, soft nice dark brown hair that faded a little bit of black, and beautiful purple eyes. The other had s/c skin, also soft nice, and short h/c, plum red eyes that faded with black on their right, and beautiful e/c eyes that faded with black on their left. The father would take a liking to the boy and hoped that he would take over the household one day. For his other child, he pretty much despised them.

The two children never knew why they were both treated differently, but that didn't stop the bond that they had together. The brother would sometimes secretly sneak into their sibling's room to spend time with them, but that would sometimes end up with the brother getting 'disciplined' for his actions. Since both of them could rarely be with each other due to their father. The siblings were both pretty sad since they couldn't spend time with each other anymore, but before they were both split apart they both made each other something just to remember one another. The brother gave them a mask. It had two eye holes, it had similar markings that the other had on their face, and it also fits perfectly. The other made them a bracelet that had an engraving that said, '永遠に一緒にそして決して離れることはありません' it meant 'together forever and never apart'(I'm sorry I just used google translate for this)

They both loved their gift, and would always keep them on. Another year passed and the brother started learning swordsmanship once the family started hearing about demons. While the brother would practice, someone would be watching him, catching everything that they were doing, and talking about. Ever since the child would secretly teach themselves how to use a sword. About a month later the h/c child was able to use the sword professionally.