Servant Summoning System in One Piece

"Servant Assassin has been summoned. I ask of you. Are you my master?" "Yup. I'm Gol D. Sou, nice to meet you!" In which a teen assassin who isn't quite right in the head is reincarnated as the oldest child of Gol D. Roger and decides to live a free life along with his Servants from the Fate franchise.