"I thought he was a devil fruit user" Commented Vivi after watching the swordsman jump into the sea to rescue their captain. "Oh no, he is not a devil fruit user, he is the devil" Explained Nami and oh well. "Oh I see, wait- " . Or Zoro is an actual demon, and they are going to be the greatest swordsman in the world - even if they don't shit about swords - oh and Luffy kind of made a deal with him.