Kaosu, known as The Ancient One, is a legendary warrior mage who defeated the demon lord Talpa a millennium ago, halting Talpa's conquest of the mortal world. After this victory, he shattered Talpa's armor into nine distinct sets, each imbued with a specific virtue, and entrusted these armors to nine families to safeguard against Talpa's return.

He serves as mentor and spiritual guide to the five warriors inheriting the primary armors, imparting mastery over their elemental powers and virtues. His training emphasizes the philosophical principles—righteousness, justice, wisdom—linked to their armors, crucial for their unity against Talpa's forces.

Kaosu is revealed as the original bearer of the white Armor of Inferno (Kikoutei), an armor embodying the convergence of the five virtues and possessing immense power. This legacy passes to the protagonist, Ryo Sanada, who becomes its new bearer.

His story details a profound sacrifice to protect the mortal world, leading him to transcend his physical form and become a spiritual presence. From beyond, he continues to guide the warriors, manifesting in visions or critical moments to offer wisdom and warnings.

Kaosu is intrinsically linked to the Jewel of Life, a powerful artifact capable of sealing Talpa's power and shielding the mortal realm. This jewel becomes pivotal later, protecting key allies like Mia and Yulie. His essence resonates within the jewel, reinforcing its protective capabilities.