A-ô is a character who serves as an S-Class ES member, a secret agent operating under the Galactic Organization of Trade and Tariffs, known as GOTT. He is a young man of nineteen years who presents a striking physical contrast to many of his peers, being a giant of a man with black skin. His imposing and formidable exterior, however, belies a nature that is notably gentle and principled. A-ô is a pacifist at heart, a person who places a high value on abstract concepts such as justice and honor, and he strives to act according to his own firm convictions. His demeanor is consistently courteous and respectful when dealing with others, and he has a particular fondness for sweet foods, a preference that seems incongruous with his massive build. He also has a tendency to express himself in long, sententious phrases, giving his speech a profound and sometimes preachy quality.

A-ô operates in a partnership with Un-ô, a relationship that predates their formal enrollment in GOTT. The two share a bond of the most absolute trust, and while Un-ô may be more volatile, A-ô functions as the more stoic and level-headed half of the duo. Before becoming ES members, they worked together as mercenaries, and during that time, they had several confrontations with the prominent agents Éclair and Lumière, establishing a rivalry that continued after all of them joined the organization. In the course of the story, when Éclair and Lumière rebel against GOTT, A-ô and Un-ô are initially dispatched to deal with them, but they ultimately make the same choice to betray the organization themselves, later going into hiding before re-emerging to support the character Eclipse.

The unique ability possessed by A-ô is called Amplifier. This power primarily manifests as an enhancement of his visual perception, allowing him to see the flow of vital energy, or life force, that is emitted by all living beings. He can track this energy, which vanishes upon the death of the creature, making it an invaluable tool for hunting enemies. Furthermore, A-ô can amplify his own nervous system, using this same principle to dramatically boost his overall physical abilities, such as speed and strength, in combat. He also works in tandem with his partner, using the energy that Un-ô perceives to further increase the destructive power of his physical attacks. During a major attack on GOTT headquarters by a group known as the G Society, he fought fiercely to defend it. Following the forced disbandment of GOTT and the termination of his role as an ES member, A-ô still chose to participate in a later, critical battle, joining with Un-ô to fight with all his strength in a desperate effort to save a space vessel called Erdemeteol.