The ranks within the Marine System are fairly straightforward, where each rank is clearly delineated.
Officers present at Marine Headquarters are considered the elite forces of the Marines.
“Fleet Admiral” is the highest Marine rank, and is the commander of the entire organization. It is just above admiral but below the World Government Commander-in-Chief and the Gorosei. The fleet admiral and the three admirals are the only ones within the Marines capable of legally initiating a Buster Call, as well as granting World Government agents or lower ranking Marines the right to do so. The same also applies to the revocation of a Shichibukai title.
Admiral” is the second highest rank in the Marines. There are only three Marines who hold this title at any given time, and they are crowned as the World Government’s “Greatest Military Powers” as each of them possess deadly combat abilities. They are the only Marines, along with the fleet admiral, able to issue a Buster Call on any island they deem a threat.
“Vice Admiral” is the third highest rank in the Marines. Vice admirals are well known for their strength, and it would seem that most of them are swordsmen and several have been shown using Rokushiki. Another attribute that is common among vice admirals is that they all know and can use at least one type of Haki.
“Rear Admiral” is the fourth highest rank in the Marines, and is the first (and the lowest) admiral rank. “Commodore” is the fifth highest rank in the Marines.
“Captain” is the sixth highest rank in the Marines. One of the duties that a Marine captain could have is to be the commander of one of the several Marine branches established on the islands around the world. A Marine captain is considered a strong Marine officer whose experience and battle performance are forces to be reckoned with. Most of the several Marine captains seen in the series are powerful fighters with Devil Fruit powers, high combat skills or considerable tactical knowledge.
“Lieutenant” is the ninth highest rank in the Marines. “Lieutenant Junior Grade” is the tenth highest rank in the Marines. “Ensign” is the eleventh highest rank in the Marines, and the lowest and last rank within commissioned officers. This is the lowest rank given permission to wear the kanji “Justice” on the backs of their clothing.