Aaron is the main antagonist of the series, serving as a fallen god whose actions drive much of the conflict in the story. He was originally one of the divine beings known as the Eight Pillar Gods, holding the position of the God of Amusement. In this role, his responsibility was to provide entertainment for humanity living in the fantasy world. However, as time passed, he grew weary of this duty. His boredom eventually drove him to madness, and he turned to sadistic methods for his own amusement, deciding to force humans into elaborate death games. This twisted entertainment came at the expense of widespread suffering, as he manipulated several kingdoms into full-blown wars and turned parts of the world into his personal playground.

Aaron possesses a deeply sociopathic and sadistic personality. He is willing to do anything to relieve his own boredom, showing no regard for the lives or well-being of others. His cruel nature is reflected in his appearance, as he is fair-skinned with black hair and blue eyes that feature distinctive diamond-shaped pupils. He typically wears a black cloak over a blue shirt with a collar, and he is often seen with a wild, murderous grin that perfectly matches his character. He also gives off a purple aura that indicates his immense power.

Because of his actions, the other gods were forced to intervene. They summoned heroes from another world to stop him. Among these heroes was Yuuya Hirosawa, who would go on to become the first ruler of the Kingdom of Esfort. Yuuya, alongside others, fought against Aaron and managed to seal him away inside a crystal ball. However, this victory came at a great cost, as Aaron killed the parents of the protagonist, Cain von Silford, during the confrontation.

Though sealed, Aaron's presence looms over the entire series. The shards of the crystal ball containing him have been dispersed throughout the realm, and his influence continues to be felt. The gods empowered Cain as a means of training him to eventually fight Aaron, as the seal is slowly eroding over the centuries. Even in his sealed state, Aaron can create apostles using the jewel fragments. One such person was Corzino, who became an apostle during events involving the Republic of Ilstin. Through Corzino's severed head after his death, Aaron spoke to Cain for the first time, becoming aware of his existence. On another occasion, when a demon lord called Agus ingested a large jewel fragment, Aaron was able to temporarily possess his body and fight a short but one-sided battle against Cain, demonstrating that larger fragments release more of his power. In the manga and anime, an early glimpse of this occurs when a black dragon swallows a small fragment, leading to an altercation with Cain that alerts Aaron to the young hero.

His goal is to break free from his prison completely. To spite the gods who banished him, he tries to raze the Kingdom of Esfort to the ground by driving the monsters in the forest into a savage frenzy and sending them to demolish the kingdom. He also sadistically tortures Cain's familiars when given the chance. Because Aaron was sealed away, the world suffers from a shortage of entertaining things, such as board games, making the restoration of amusement a task that Cain is later entrusted with.