In the world of One Piece, Fleet Admiral (元帥, Gensui) is the highest marine rank, and is the avoided commander of the entire organization. It is just above Admiral but below the World Government Commander-in-Chief and the Gorousei.
The only Fleet Admirals seen so far in the series are Kong from 27 years ago, who was eventually promoted to the rank of World Government Commander-in-Chief, and Sengoku, who was Fleet Admiral until he resigned from his post three weeks after the Battle of Marineford. Sengoku suggested to Kong that Admiral Aokiji be his successor as Fleet Admiral. In between the two years where the Straw Hat Pirates took a training break, a new Fleet Admiral is mentioned, but yet to be named.
The World Government Commander-in-Chief, 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.
Fleet Admiral Sengoku
Former Fleet Admiral Sengoku (センゴク元帥, Sengoku Gensui) was the supreme commander of all the Marine forces in the world, his status apparent by the life-size seagull attached to his hat. He was one of the major figures along with Edward Newgate, Shiki, and Monkey D. Garp while Gol D. Roger was still alive, and still continues to be in the present, even after his resignation from the Marines.
Sengoku is a tall and muscular, yet spry, roughly middle-aged man with a long braided beard and a mustache. He is usually seen alongside his pet goat. He typically wears glasses and a full Marine Admiral uniform that is adorned with medals. The most distinctive features of this uniform are a life-size seagull on top of his cap and his oversized Marine coat, which he wears like a cape. His cap covers his hair, which is set in a large afro. When he uses his powers, his hat is knocked off, revealing his afro.
Twenty years before the present storyline, when Sengoku was already an Admiral, he did not cover his afro and had a beardless face. During his orders to Saul to bombard Ohara, he wore a less-regal attire than he does in the present storyline, a simple black tank top, blue jeans, and his glasses, which is unlike a typical Admiral’s formal suit; the only Marine related attire he wore at the time apparently was his Marine coat, draped over like a cape. However, when he was fighting Shiki in Marineford, he was wearing a black suit under his coat.
Sengoku appears to be a proud man who is loyal to the World Government and, up until the cover-up of the Impel Down incident, never questioned orders. He also won’t accept any reason for letting a criminal go and also has a habit of getting very irritated with failure. While it has not been revealed what kind of justice Sengoku follows, he certainly does not shy away from actions that falls under the code of Absolute Justice. From his view, the Shichibukai are just pirates, though he notes how valuable they can be when the time comes. However as the years went by, he appears to be losing his faith in the government, shown after the Whitebeard War when he was angry at the order from the government to cover up the break-out incident in Level 6 of Impel Down to save face and keep the faith of the people, thus placing nations in danger. This eventually led Sengoku to resign from the Marines.
As of late, it would seem that he has become concerned over the movements of the Straw Hat Pirates, and has followed their movements ever since they left Water 7, though at the time his current concern was about the upcoming war against Whitebeard, to the point that he brushed off the incident of the attack of a Tenryuubito and left it to the Admirals. Upon hearing about Luffy’s break-in of Impel Down, Sengoku was so frustrated that he almost took it out on Garp, and had come close to losing his patience with the antics of the Monkey family in general.
Sengoku also believes that a person’s birth (with the consideration of the parents’ reputation) is enough to determine the potential impact they could bring to the world if left unchecked, such as Ace being Roger’s son and Luffy being Dragon’s son. This is shown at Ace’s execution where one of the main reasons for Ace’s execution was not because he was a pirate, but because he was the son of Gol D. Roger and he believed that Ace must be executed by any means necessary even if that meant an all out war with Whitebeard.
His epithet Sengoku the Buddha may seem ironic at best because through the course of the series, he has shown himself to be an extemely calm, composed, collected man, he has also shown himself to be quite capable of violence. Doflamingo remarked on this irony when the Shichibukai convened at Mariejois to chose a successor for Crocodile. However, with the revelation of his Devil Fruit, it would seem his epithet’s origins come from his fruit’s ability, exactly like the three current Admirals. He also appears to have some common sense, as shown when he ordered the Whitebeard War to be over and ordered for people to tend to the wounded. His Justice isn’t as uncompromising as Akainu’s; it seems to be more of a mix of both absolute and moral justice, as he did make a large speech about morality. He also called the war to an end upon Ace and Whitebeard’s demises, after Shanks made his appearance. An another hint of Moral Justice is also shown when he got infuriated when the World Government decided to cover-up the break-out of the Level 6 Impel Down prisoners released by Blackbeard, although he once said not to question the World Government‘s orders.
As the commanding officer of the entire Marine force, Sengoku had absolute power over every action the Marines do, including summoning a Buster Call. The only ones that are above him and command him are the Gorousei and Commander-in-Chief Kong. However, with his resignation came the loss of such authorities.
24 years ago, when Shiki was causing havoc in Marineford hearing about the capture of Roger, Sengoku fought alongside Garp and together, they were powerful enough to subdue him, whilst destroying half the city in the process, and also keeping him imprisoned in Impel Down. Even without using his Devil Fruit powers, it is implied that Sengoku also has superhuman strength on his own, enough to hold down an enraged Garp.
He is also a military commander of the highest order, as demonstrated at Marineford. He is shown to have organizational and tactical skill, as his “Squardo plan” resulted in Squardo critically injuring Whitebeard and his “Siege Wall Plan” combined with Admiral Akainu’s Meteor Volcano has succeeded in routing the Whitebeard Pirates and their allies, which led to the destruction of the crew’s flagship, the Moby Dick. He was referred to as “Strategist Sengoku the Buddha”, an apt name.
It was stated that all Vice Admirals possess Haki, so as a former higher ranked officer as well as leader of the Marines, Sengoku can also use the ability. During the Battle of Marineford, he used Busōshoku Haki to punch Monkey D. Luffy (while using Gear Third) in the stomach, making him cough blood.
Sengoku ate the “Hito Hito no Mi, Model: Daibutsu”, a rarer-than-Logia Mythical Zoan-class Devil Fruit which allows him to turn into almost like Buddha and becoming similar to a gigantic Daibutsu, which is probably the reason for his nickname. Nearby Marines claim that they have never seen him use this power before the war.
While enlarging, his Marine jacket, glasses, and pants enlarge with him; it is unclear what happens to the rest of his uniform. The color of his body (and uniform) seems to change to a dark color, appearing to be ominously shadowed at all times. In this form his body has different proportions, his torso and arms are much larger compared to the rest of his body.
This ability gives him a massive power boost to his already impressive strength, enough to cause the execution stand to collapse in one indirectly connecting punch, and that despite Luffy’s Gomu Gomu no Gigant Fuusen which had absorbed the brunt of the attack.