Akihiko Kayaba, creator of the virtual reality game Sword Art Online and the NerveGear device, displayed an early preference for books over games or computers during childhood and participated in his high school Mathematics Club. He frequently worked in university laboratories without engaging in social circles. Kayaba developed the floating castle concept that became SAO's Aincrad setting. His primary collaborator, Koujiro Rinko, assisted him for ten years during development and later cared for his physical body during extended virtual dives. On November 6, 2022, Kayaba trapped approximately 10,000 players inside SAO by disabling the logout function. Using a Game Master avatar—a large, faceless hooded figure in a red cloak—he announced the death game rules: players dying in-game or having their NerveGear forcibly removed would suffer fatal brain damage. He revealed all players' avatars reflected their real-world appearances except his own, the idealized Heathcliff persona. Heathcliff possessed a muscular physique, steel-gray hair, and sharp scholarly features, contrasting with Kayaba's slender build. As Heathcliff, he founded and led the Knights of the Blood guild, becoming the game's strongest player. He wielded the sword-and-shield set Liberator and possessed the unique Holy Sword skill, granting near-impenetrable defense. Rumors claimed his hit points never entered the yellow zone, aided by system-assisted immortality activating below 50% health. He earned the title "Man of Legend" after single-handedly engaging the 50th-floor boss for ten minutes. Kayaba maintained a dual role throughout the death game. As Heathcliff, he participated in frontline raids, including the first-floor boss battle alongside players like Kirito and Asuna. He delegated guild management decisions but intervened when Asuna attempted to leave the Knights of the Blood, challenging Kirito to a duel and recruiting him after victory. His true identity remained hidden until Kirito exposed him during the 75th-floor boss raid. Kayaba then dueled Kirito under modified conditions: Kirito's victory would free all surviving players. Despite activating his immortality, Kayaba accepted defeat after Kirito and Asuna overcame the game's mechanics through willpower. Following SAO's clearance, Kayaba used a modified FullDive machine to perform a high-powered brain scan, transferring his consciousness into the network and destroying his physical body. His digital consciousness later surfaced during the Fairy Dance arc, aiding Kirito against Sugou Nobuyuki in ALfheim Online by providing administrative credentials. He entrusted Kirito with the World Seed program to create new virtual worlds. In the Alicization arc, Kayaba's consciousness monitored events from within the Ocean Turtle facility's systems, intervening to prevent a nuclear meltdown by controlling a maintenance robot and sacrificing his data integrity to stabilize the reactor. During the Unital Ring arc, he secretly communicated with the Star King, a digital incarnation of Kirito from the Underworld. Kayaba exhibited calmness and humility, accepting defeat gracefully and expressing genuine interest in Kirito's growth. He maintained a strict sense of fairness, designing SAO as a beatable game and refraining from using Game Master privileges to hinder progress. He actively aided in clearing floors, only activating his immortality to survive until his planned revelation as the final boss on the 100th floor. However, he showed minimal empathy for the consequences of his actions, treating player deaths dispassionately and expressing amusement when Asuna was seemingly killed during his duel with Kirito. His primary drive was realizing his childhood dream of a floating castle, disregarding ethical boundaries to achieve it.