Furina

From an ontological and genealogical perspective, Kaeya is a human of pure or near-pure Khaenri'ahn descent belonging to the Alberich Clan. As revealed during the Caribert Archon Quest through the encounter with his ancestor Chlothar Alberich—the founder of the Abyss Order—and corroborated by Dainsleif, the Alberich family was a noble house of Khaenri'ah that stepped up as Regents when the Eclipse Dynasty ruler King Irmin became incapacitated prior to the Cataclysm five hundred years ago.

The physical hallmark of Kaeya's Khaenri'ahn bloodline is visible in his exposed left eye, which displays the unmistakable four-pointed diamond-star pupil native to citizens of the underground kingdom. While Kaeya tells gullible acquaintances that his right eyepatch hides a dramatic piracy wound or inherited equine bad luck, childhood correspondence and his own history confirm he originally wore the patch over a scarred or concealed right eye from the time he arrived at Dawn Winery, and his Mondstadt Cryo Vision uniquely features only two pairs of wing feathers on its frame instead of the standard three.

On a torrential afternoon when Kaeya was still a young boy, his biological father brought him to the edge of the Dawn Winery vineyards in Mondstadt, squeezed his shoulders, and whispered a cryptic mandate before vanishing forever to fetch 'grape juice': 'This is your chance. You are our last hope.' Left shivering in the pouring rain, the abandoned child was found and taken in by Master Crepus Ragnvindr, the compassionate owner of Dawn Winery, who adopted Kaeya into his household and raised him alongside his own biological son, Diluc.

For years, Kaeya and Diluc grew up as inseparable sworn brothers, joining the Knights of Favonius together and rising rapidly as the order's twin prodigies: while Diluc fought in the gleaming spotlight as Cavalry Captain, Kaeya served as the brains behind the scenes. However, this idyllic brotherhood shattered on the day of Diluc's eighteenth birthday when the shadowy monster Ursa the Drake ambushed their convoy and Master Crepus died after unleashing a forbidden Delusion to save them.

Consumed by overwhelming guilt for having secretly rejoiced for a split second that Crepus's death freed him from the agonizing choice between Khaenri'ah and Mondstadt, Kaeya knocked on Diluc's door that rainy night and confessed his entire secret identity as an agent of Khaenri'ah. Enraged by the betrayal in the wake of their father's death, Diluc drew his flaming greatsword; at the exact instant their blades clashed, Kaeya's Cryo Vision manifested in a burst of frost that blocked Diluc's Pyro strike, leaving the two brothers estranged ever since.