This one will be very dark.
There will be KakuHida, bit not yet.
Glasgow smile (plural Glasgow smiles)
A facial scar received from being cut on the edge of the mouth, resembling an extended smile.
When Hidan was little he was not aware that he grew up in a bad neighborhood. Sure, he knew not to walk the streets at night, he knew not to make the older boys angry, he knew that stuff.
But not until that girl with the smile did he realize what the world was really like.
She had been a friend of his as a child, and they wold often play together. They were neighbors, and Hidan liked her a lot. She was his first, and for along time, his only friend.
She had a beautiful smile. He liked that the best.
Then one day when they were both 13 she had been out alone one night, getting milk, and didn't come home until late. Hidan knew this because he heard the ambulance and saw the flashing lights, and prayed to Jashin that she was alive.
He didn't fully understand what happened to her back then. She was attacked. She was hurt. They sliced her face open, two long extensions of her mouth reaching up to her ears.
(They said "What a nice smile you have pretty girl. Why aren't you smiling now, with us? We'll make you smile.")
But Hidan didn't understand what all happened back then.
He had seen her later, her face scarred. She cried when he saw her.
"I'm so ugly now!" she cried, tears dripping down her ear to ear smile. "I'm hideous!"
Hidan didn't say anything. What would he say? She obviously couldn't understand how lucky she was, to be able to smile like that.
6 months later she killed herself. She slit her wrists, just like the boys slit her face.
The identity of the girl is unimportant, so if you would like to picture her as any specific character, go ahead.
Preemptive strike: This may have an unhappy ending. You never know.