Sakazuki, known almost universally by his alias Akainu, is a high-ranking officer in the Marines of the World Government. He stands as a formidable figure who, over the course of the story, rises from the position of an admiral to become the fleet admiral, the highest operational authority in the entire organization. His birth took place in the North Blue, and he is a large, muscular man of great stature, with short black hair and amber eyes. In his youth, he was depicted wearing a hooded shirt and already carrying a sword, and his appearance has changed over time, notably after the timeskip he grew a beard and began smoking a cigar, with his face bearing new scars from a past battle.

The core of Sakazuki's identity is his unwavering and extreme interpretation of justice, which is often referred to as "Absolute Justice" or more specifically "Thorough Justice". He operates on a philosophy that the complete extermination of evil, by any means necessary, is not only justified but required. This leaves no room for compromise, sentiment, or collateral damage. His demeanor is consistently stern, grim, and dead serious, reflecting a personality that is the most ruthless among his former admiral colleagues. He holds immense pride in the name and reputation of the Marines, believing any action that tarnishes their image is a grave offense.

His motivations are driven by this radical belief system. For Sakazuki, the concept of "evil" is not limited to actions but extends to lineage and potential. He famously declared that crimes are inherited, which is why he considered Portgas D. Ace, as the son of Gol D. Roger, and Monkey D. Luffy, as the son of the revolutionary Dragon, to be inherent threats that had to be eliminated regardless of their personal choices. This logic leads him to justify horrific acts in service of a perceived greater good. He has no tolerance for cowardice or dissent, even from his own forces, believing any Marine unwilling to sacrifice their life for justice is unworthy of the title.

In the story, Sakazuki serves as one of the primary antagonists, particularly during the Marineford Arc, and his actions have long-lasting consequences for the world. His most infamous role was during the Summit War of Marineford, where he was a central figure in the defense against the Whitebeard Pirates. He employed deceptive tactics, manipulating one of Whitebeard's allied captains into betraying and stabbing the emperor. He was relentless in combat, using his powers to inflict massive destruction on both enemy pirates and deserting Marines. The defining moment of the battle was his killing of Ace. After goading Ace into a fight by insulting Whitebeard, he delivered a fatal blow that has haunted Luffy ever since. Following the war, he entered a ten-day duel with his colleague Aokiji for the position of fleet admiral, a battle he won, transforming the climate of Punk Hazard into a landscape of fire and ice as a testament to their clash. As fleet admiral, he later moved the Marine headquarters to the New World and implemented a worldwide draft to bolster their forces.

His key relationships are defined by his ideology. He shares a particularly strained relationship with his former comrade, Aokiji, who opposed his promotion and found his methods to be excessive, ultimately resigning from the Marines rather than serve under him. He holds little respect for legendary figures like Monkey D. Garp, seeing him as part of an older generation. He views Luffy with an obsessive hatred, seeing him as the son of the world's worst criminal, and is personally responsible for the large X-shaped scar on Luffy's chest. His leadership is feared and respected by his subordinates, and even powerful pirates like Blackbeard have chosen to flee rather than confront him directly.

Sakazuki has shown some development over time, transitioning from an admiral who operated on the front lines to a fleet admiral who must manage the entire organization from headquarters. While his core beliefs have not softened, his position has forced him to consider broader strategic implications, such as the political fallout of his subordinates' actions and the global balance of power.

His notable abilities are formidable. He ate the Magu Magu no Mi, a Logia-type Devil Fruit that allows him to create, control, and transform into magma at will. This power is considered to have one of the highest offensive capabilities, capable of melting steel and even burning fire itself, as demonstrated in his fatal confrontation with Ace. In addition to his devil fruit, he is a highly proficient user of both Armament and Observation Haki, making him an incredibly durable and powerful combatant. His authority as fleet admiral also grants him command over the entire Marine forces, allowing him to issue Buster Calls and direct global military strategy.