Kurai

He was born a frail child, abandoned, poor, but he stole a lot of food. He had every right to become a demon but he wanted to help. His goal is to live in a world where people can go out at night with out fear of demons. So he became a demon slayer but none of his comrades liked him bc he was so gloomy. He was a flame breather but then battling a upper rank his flames started acting weird, they changed color and he became stronger, faster, and more stealthy. This is Kurai the abyss hashira.