ASL Marine Sister (One Piece Fanfic)

Shiro was a 16 year old when she met Luffy, Ace and Sabo. It had only been a few months since Luffy was left with the mountain bandits so Ace and Luffy had just started to become friends and Sabo had joined them on their home when she was left there as well. Shiro was different from them, her expression was always dark and they never saw her smile at the beginning. Shiro was twisted in the head, she was a sadist, and it was understandable seeing as her parents were assassins. She was born and raised to kill and torture. But soon somehow the three little kids make her warm up to them.. when she finds out that they want to become pirates and Garp wants them to become Marines she makes a deal with him. She would become a Marine and in return he would grant the three kids the freedom to become pirates and not go after them even if he knew where they were.. what happens then? Also she has a devil fruit ability.. what is it?