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 firstmate is the next in line to take over the captains job. In history there were a few incidents where the firstmate killed the captain to take his role, cases like this were rare but not unheard.

One Piece
Most pirate crews have a first mate. From Buggy to Foxy this crew role is common to be seen. 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 firstmate is regarded as the captains champion.

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. However, Ussop states he is the First Mate in One Piece Grand Battle! which incorrect as he is the crews sniper, though a promotional video from 4Kids states Zoro to be First Mate.