A-ou is an S-Class ES Member, a secret agent working for the Galactic Organization of Trade and Tariffs (GOTT). He is nineteen years old and operates in a partnership with his colleague, Un-ou. Before joining GOTT, A-ou worked as a mercenary, which created an intense professional rivalry with the C-class ES pair Éclair and Lumière, a dynamic that persisted after all parties became members of the ES force. In personality, A-ou is stoic and pacifistic, guided by a strong sense of justice and honor. He treats others with respect and often speaks in a profound manner. Despite his imposing physical stature, he has an inconsistent liking for sweet things. His ES ability, known as Amplifier, allows him to amplify his vision to detect the flow of energy emitted by living things and perceive when this life energy fades. He can also amplify his own nervous system to enhance his physical abilities, making him a formidable combatant whose fighting style relies on powerful physical techniques. Driven by his moral compass, A-ou eventually chooses to betray GOTT alongside Un-ou, going into hiding before re-emerging to support the character Eclipse against their former organization. After the order for GOTT to disband, he is unable to function as an ES Member, but later fights with full effort alongside Un-ou in a mission to save Erdemeteol. A-ou and Un-ou have also acted as instructors to another character, Ku Piyu.