It turned out to be Suigetsu, the ghost in Naruto, who somehow ended up in the world of One Piece.
Because Shuiyue's body has not drank or been exposed to water in a day, it will lead to a serious lack of water in the body.
On the verge of death, Robin slits his own wrist, allowing Suigetsu to be reborn.
From then on, Suiyue took Robin, avoiding capture by the navy, while looking for the decapitating sword in his mind.