It's Brooke who plays the violin. Brooke joined Luffy in the 338th episode of Joy in Meeting People! The true face of a skeleton gentleman.
Brooke is a character in the comic book One Piece and its derivative works. He is known as the nasal song brook and the king of the soul. He is a musician of the Straw Hat Pirates. He can be resurrected once after his death, because the fruit has completed its mission and can't swim.