A First Mate (副船長 Fuku Senchō?, literally meaning "Vice Captain") 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.
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/woman 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. 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 of. Sometimes even, the first mates are not treated as they should and someone else would be doing their job but they would still be considered the first mate. Other times, the first person to have joined that crew will have become the first mate when the crew has formed.
The first mate's mainly responsible for the vessel's stability and cargo operations. They supervise the crew and are also responsible for the safety and security of the ship, as well as the welfare of everyone on board. Additional duties include maintenance of the ship's hull, cargo gears, accommodations, the life-saving and the firefighting appliances. The first mate also trains the crew on various aspects like safety, firefighting, search-and-rescue, and various other contingencies.
On most vessels, the Chief Officer and First Officer (or First Mate) are synonymous, but passenger vessels often carry a separate First Officer who is junior to the Chief Officer.
First Mates throughout Hedgehog Piece
Most pirate crews have a first mate. It is not uncommon to see first mates throughout crews of pirates. There are some cases where larger crews use other systems of management such as a "Division" commander, in which The Ouroboros have only been seen using, and likely as a result 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 their crew's second strongest, so to speak. In some cases this may not be exactly true.