Sakazuki, commonly known by his alias Akainu, is the current fleet admiral of the Marines, succeeding the previous owner in Sengoku. During the first half of the series, he was one of the three admirals, and the last one to be fully revealed. He ascended to the position of fleet admiral after defeating Kuzan in a ten-day duel that left Punk Hazard in its current state. One of his first decisions was to relocate the Marine Headquarters in the New World in prepartion for the New Pirate Age.
In the manga, he was first mentioned by Nico Robin during the initial encounter with Aokiji. In the anime, he was mentioned by Fleet Admiral Jonathan during the G-8 Arc, which aired after Akainu had been revealed in the manga. Because of his actions, Akainu was one of the primary antagonists of the Marineford Arc.
Sakazuki is a strident follower of Absolute Justice, to the point where his extremism prompted an equally high-ranking colleague to resign the Marines. He believes that a person's hertiage is a reflection of how big of a threat they pose - a belief which caused him to target Portgas D. Ace and Monkey D. Luffy during the Battle of Marineford for the purpose of purging their "bad blood", which ultimately led him to personally kill Ace and attempt to kill Luffy, which he failed to do - manging only to massive scar on his chest.
He is one of the main antagonists in the anime/manga One Piece.