Khun Aguero Agnis, a prominent member of the Khun Family among the Ten Great Families, is the direct son of High Ranker Khun Eduan. Expelled following his older sister's failure to become a Zahard Princess, this event triggered severe consequences: his sister's suicide and the casting out of his mother's Agnis lineage. Fueled by an inferiority complex from this fallout, he harbors the ambition to usurp his father's position as family head. Even before becoming a Regular, he demonstrated audacity by raiding Khun Eduan's treasure chamber, an unprecedented act risking grave punishment. His childhood immersed him in harsh political warfare, forcing early maturity and denying him a typical upbringing. This environment forged his guarded nature and deep distrust, while also instilling a longing for simpler, genuine experiences.

He possesses the Khun Family's characteristic silvery-blue hair, cobalt blue eyes noted to vary in shade across the story, and a pale complexion. His build is small yet muscular. His appearance evolves: Part I features shorter hair with a blue bandana and formal attire like a white shirt and black tie. Part II introduces varied outfits including safari gear and cloaks. Part III adds a small braid tucked behind his ear. Other characters frequently note his angular features and striking looks.

His personality blends cool-headed intellect with ruthless pragmatism. A master strategist, he manipulates test rules and opponents to achieve his goals, his dangerous cunning recognized even by Rankers. Despite this outwardly calculating demeanor, he possesses deep sensitivity and loyalty, particularly toward those he deems precious. This dichotomy stems from childhood trauma; amidst family intrigue, he formed a bond with his half-sibling Maria, whose kindness offered respite. Her departure left him emotionally adrift, creating a void later filled by his friendship with Twenty-Fifth Baam. He adopts a fiercely protective stance toward Baam, willing to sacrifice himself or isolate himself to ensure Baam's safety. Conversely, he exhibits vindictiveness toward Rachel, blaming her for Baam's suffering and orchestrating elaborate schemes to undermine her ambitions.

As a Light Bearer, he excels in lighthouse control, utilizing skills like teleportation fields, barriers, and system hacking. He chose this position from a preference for commanding others, not weakness. He also inherits the Khun Family affinity for ice-based shinsu, manifesting Ice Spears—a rare quality he initially resisted due to disdain for his heritage. Over time, he integrates this power with his lighthouses, enabling techniques like freezing opponents or combining forces with allies such as Rak for fusion skills. Despite physical limitations against high-tier foes, his adaptability allows rapid mastery of new abilities like Enna Core. His combat prowess includes exceptional speed, strength, and spear proficiency, though he relies primarily on strategy over direct confrontation.

His development shows significant evolution. In Part I, he allies with Baam and Rak during Floor of Test trials, showcasing tactical brilliance and forming an unshakeable bond with Baam. Following Baam's apparent death, Part II depicts his relentless pursuit of truth and vengeance against Rachel, forming elite teams to control her ascent while masking grief with cold efficiency. A critical moment occurs in the Hidden Floor where Khun Kiseia stabs him, and Rachel implants a bomb in his body. He survives by freezing his own organs, entering a coma later healed by Yeon Family flames. This ordeal leaves him with an internal "fire fish" entity and renewed resolve. By Part III, he accelerates his climb to rejoin Baam, demonstrating refined shinsu control and unwavering commitment to their shared goals. Throughout, he balances calculated manipulation with profound loyalty, embodying a complex drive for power intertwined with a desire to protect the few he cherishes.