First Mate

A First Mate is the top officer on board a ship after the Captain. Mostly, they may fill a number of roles from checking stock to making sure everyone else is doing their job on the ship. He answers only to one person on a ship, his captain.

Essentially, the first mate is the one officer the captain must put full trust in and pick carefully. He is the captain's right-hand man and if the captain is not at hand on the ship or something has happened to him, the First Mate is the next in line to take over the captain's job. In history there were a few incidents where the First Mate killed the captain to take his role, cases like these were rare but not unheard.

One Piece
Most pirate crews have a first mate. From Buggy to Foxy this crew role is a common sight. There are some cases where larger crews have used other systems like "Division" commander as seen in Whitebeard's crew, prompting a hint of the sheer size of the crew. While the captain is regarded as the strongest crew member, the First Mate is regarded as the captain's champion, or second strongest, so to speak.

On the Straw Hats, many people believe this position belongs to Zoro as Luffy refers to him as his champion and partner early on in the series, also considering he fills no other role. Seeing that Brook was appointed captain of the surviving half of the Rumbar Pirates, he might have been first mate of his old crew. Also, Bepo of the Heart Pirates seems to have command over the newcomers to the crew, so he might be first mate.

List of Known First Mates

 * These do not include commanders of "divisions", such as on the Whitebeard Pirates.
 * Ben Beckman (Red-Haired Pirate Crew)
 * Mohji (Buggy Pirates)
 * Jango (Black Cat Pirates)
 * Gin (Don Krieg's Pirate Armada)
 * Hatchan (Arlong Pirates)
 * Sarquiss (Bellamy Pirates)
 * Silvers Rayleigh (Jolly Roger Pirates)