Who is Luffy's first mate?
Well Mugiwara no ichimi has no first mate but according to One Piece Yellow: Grand Elements, Roronoa Zoro is referred to as being "like the vice captain". Bartolomeo has also referred to Zoro as the first mate of the Straw Hat Pirates, although, by his own admission, that was just an assumption based on his personal views of the crew.

link here https://onepiece.fandom.com/wiki/First_Mate