Admiral (大将, Taishō) is a title of a signified senior command rank in the Marines. Admirals, also known generally as flag officers, were typically responsible for a high level military command at battle group or fleet level, and/or for administrative and political duties relating to Marine operations. An Admiral is senior to a Vice Admiraland junior to the Fleet Admiral.
Marine Admirals in One Piece
In the series, Admirals are shown as being the second highest rank in the Marines behind the Fleet Admiral. There are only three Marines with the title of Admiral. The three current ones are known by their aliases: "Aokiji" (Blue Pheasant), "Akainu" (Red Dog), and "Kizaru" (Yellow Monkey). They are also possible behind the Yonkou, the three most powerful persons in the world of One Piece.
The Admirals are crowned the World Government's "Greatest Powers" (最高戦力Saikō Senryoku?), and are the strongest individual fighters within the World Government and the Marines. These three admirals are respected and feared amongst the entire world. As seen in Sabaody Archipelago, the mere presence of just one Admiral was enough to cause a panic on the island and make the entire population flee from the island, including the Eleven Supernovas. Another example of the Admiral's reputation is Nico Robin from the Straw Hat Pirates, related to her childhood trauma, she constantly loses her grip and starts panicking whenever an Admiral is around.
The current three Admirals all have Logia-type Devil Fruitpowers representing their respective color. All of their powers are very destructive, even when comparing to an average Logia user. They are all strong enough to combat against Whitebeard alone, as well as to fight on par with the top three Whitebeard Pirates commander each (although Kizaru and Aokiji's opponents were distracted and Akainu's magma has a natural superiority to Ace's fire). Admiral Aokiji and Kizaru are seen to be proficient swordsmen when creating blades from their own element and wielding them. It is unknown if Akainu also shares this ability, though, as he has only been seen using molten fists for close combat. In addition, the three of them are high-level users of Haki, shown when using Busōshoku Haki together to block Whitebeard'sshock wave.
Aokiji: This only happened because the encircling wall is not ready.
Akainu: Your damn ice is blocking it!
Kizaru: Why couldn't you just melt it, Sakazuki?
— The three admirals, after veering off Whitebeards attack