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. They answer only to one person on a ship, the captain.
Role and Duties
Essentially, the first mate is the one officer the captain must put full trust in and pick carefully. They are the captain's right-hand man and if the captain is not at hand on the ship or is otherwise incapacitated, the first mate is the next in line to take over the captain's job.