In his first appearance, Aeon had taken on the form of a humanoid lizard-like creature with yellow scales and red and black highlights. His only clothing was an armored breastplate, arm bands and shin guards. Upon returning in Soulcalibur III, his appearance transformed drastically, resembling an iguana-like creature with feathers growing out of various parts of his body, including his tail bangs of hair on the sides of his head and his scales have changed to a cyan color. He had long since discarded his Greek armor in favor of war-oriented gear. By this time, while he had retained his general fighting style, he replaced his holy weapons with another shield and an axe.
In Soulcalibur IV, he changes again to resemble something like a frilled-neck lizard with fins growing out of his legs, spine and tail areas and his scales have changed into a combination of beige and purple. Upon his return in Soulcalibur V, he turns into a form similar to the one in Soulcalibur III, but gains a drastic change in the form of a pair fully grown and functional wings to complete a more raptor/bird like appearance. However, due to the added fact that he is now referred to by his real name may indicate much more than a physical transformation.
The Spartan soldier, Aeon Calcos, like Sophitia, was one of the twenty-four warriors to receive an oracle from the god Hephaestus to destroy Soul Edge–one for every letter of the Greek alphabet. Armed with the holy Xi Sword & Game Shield forged for him, he set off on his journey. However, he became lost in a vast desert and eventually collapsed. A group of kindly desert travelers rescued a dehydrated Aeon and brought him to their village, and he was about to repay them by teaching them swordplay. He was unable to exchange their kindness, however, as the Evil Seed rained down and temporarily changed Aeon into a mindless killer, causing him to slaughter everyone in the village.
Kunpaetku, the grand priest of the cult order Fygul Cestemus, eventually heard stories of him and captured Calcos so that he may be used as a human guinea pig in a terrible experiment, causing him to transform into a lizardman —the first of many lizardmen to come. Calcos was ordered to bring Rock's adopted son, Bangoo, to Europe so that his soul could be stolen. The creature was eventually tracked down and defeated by Rock for the kidnapping, while aiding Astaroth in pursuing Soul Edge.
Soul Edge was struck down moments later, the energy emitted from Soul Calibur releasing Lizardman from his brainwashed state —although he remained in his transformed body. Hidden from the world, Lizardman set out on a journey to restore his human form, however upon realizing he had no place to return to, he wondered angrily why Hephaestus had not helped him in his time of need. "My god has forsaken me," he thought, before casting away the sword and shield he was given, and, bearing a new shield and a war axe, decided to find Soul Edge and destroy the god of forgery with the very weapon he feared most. This would be his new resolve and the ultimate revenge.
He possibly has a family in his homeland (according to the information in Soulcalibur III), but they are no longer important to him.
Overtime, Lizardman slowly, but gradually, lost his human feelings and memories, until finally, even his own heart had become one that belonged to a monster. Before he knew it, other lizardmen surrounded him, and with his superior strength and battle technique he dominated the group. Soon, he became their leader, leading them on violent rampages.
But at night, he would have dreams revisiting the feelings and memories of being human. He didn't even know what he was anymore. All he felt was emptiness. Suddenly, he realized that he was missing his soul. As he wondered of what has become of it, another memory struck him. His soul was inside Soul Edge. He now searches for the evil sword to obtain his lost soul.
Years later in despair at the loss of his humanity, Aeon eventually surrendered to his beastly instincts: He walked on all fours and devoured anyone —or anything— that crossed his path.
Many years passed, until one day Aeon heard an inhuman voice speaking to him inside a cave.
"Devour all, and your wish shall surely come true."
From that moment on, the beast-man gained the power to inherit the nature of those he consumed. Each time he tasted the blood of new prey, his body and soul were transformed.
Gradually he regained the power of reason. Aeon searched the faint remnants of his memory...
"How can I regain that which I have lost?" he wondered.
But he knew the answer: He must feed on new victims. He must consume the flesh of holy warriors..