In the manga, he was first mentioned by Nico Robin during the initial encounter with Aokiji. In the anime, he was mentioned earlier by Vice Admiral Jonathan during the anime-only 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.
Akainu is a strident follower of Absolute Justice, to the point where his extremism prompted a colleague to resign from the Marines. During his tenure as an admiral, he participated in the Battle of Marineford and arguably contributed the most to the Marine victory; his actions include manipulating Squard into stabbing Whitebeard, destroying the Moby Dick, and single handedly accomplishing the main objective of the campaign by executing Ace. He is also responsible for giving Luffy the massive scar on his chest.
Akainu is an incredibly tall, fair-skinned, unusually muscular man (the most muscular of the former admirals) about as tall as his fellow admirals, with a square shaped face, very pronounced facial features and short black hair. He usually wears a standard Marine cap and sports a crimson, double-breasted suit, decorated with what appears to be a pink rose on his left buttonhole. Unlike many high-ranking Marines, he does not wear a tie, instead leaving his light colored flower-patterned shirt unbuttoned showing his muscular neck and part of his tattoo. He wears black gloves and shoes. Although not canon, the anime shows his height was somewhat taller than the 301 cm tall (9'8") Jinbe.
The tattoo on the left side of his chest is black (light-blue in the anime), it contains numerous images of flowers integrated into its design. Akainu's tattoo has flame-shaped edges and it extends from the admiral's left shoulder to his hip, covering part of his back and ribs. He has another, smaller tattoo in the shape of a black sword on his right deltoids and a small part of the biceps. In the anime, his eye color is seen to be brown and this was also shown in a colored drawing by Eiichiro Oda.
In his youth, Akainu wore an open red shirt and carried a Katana on his back.
Twenty-two years ago, at the Ohara Incident, he wore a simple white Marine cap under a dark gray hood that seemed to be a part of a cloak he wore beneath his suit, or Marine coat. Until getting off his chair in order to destroy an incoming iceberg during the Battle of Marineford, his face was always hidden by his cap.
As a child, Akainu still had the grim disposition he retained as an adult. He also wore tattered clothes and carried a dagger. He wore a cap similar to the Marines' cap he would don in the future, except it has the kanji for "justice" instead.
Over the timeskip, Akainu grew a mustache and goatee. In One Piece Film: Z and Wake up! his attire is identical to the clothing he wore before the timeskip, however in the manga, his attire is relatively unseen, besides the fact that he wears the same floral pattern shirt he wore two years prior.