Kazuto Kirigaya, known in-game as Kirito, was born Narusaka Kazuto on October 7, 2008. After his biological parents died in an accident, his aunt and uncle, the Kirigayas, adopted him. Raised alongside his cousin Suguha, he began kendo training at age eight under his grandfather’s enforcement. Kazuto abandoned kendo after two years, drawn to computers by his aunt’s work as a computer magazine editor. His grandfather’s physical punishment for quitting led Suguha to intervene in his defense. At ten, Kazuto discovered his adoption through erased family records, unintentionally distancing himself from Suguha. Kazuto joined a thousand beta testers for the virtual reality MMORPG Sword Art Online. On the game’s November 6, 2022 launch, he was among ten thousand players trapped by creator Akihiko Kayaba, who linked in-game death to real-world fatality. During the first floor boss battle, Diavel perished due to beta version discrepancies, inciting hostility toward testers. To divert this, Kirito falsely claimed skilled testers withheld information, adopting the label "Beater" (Beta Cheater). This defined his early solo playstyle, though he aided as a clearer in boss raids. Victorious, he claimed a black leather trench coat as his signature attire. In Sword Art Online: Progressive - Aria of a Starless Night, expanding the first floor, Kirito rescued Asuna Yuuki from monsters. He invited her to a strategy meeting for the floor boss, Illfang the Kobold Lord. Collaborating with Asuna and her friend Mito during the battle, he defeated the boss after Diavel’s death. Players accused Kirito of withholding critical information, cementing his "Beater" identity. He secured the coat reward and ascended to the second floor with Asuna. Kirito briefly joined the Moonlit Black Cats guild while hiding his high level. A fatal dungeon error annihilated the guild, traumatizing him with guilt—especially over Sachi’s death. This intensified his solitude until his bond with Asuna offered solace. He married her in-game and joined the Knights of the Blood after losing a duel to leader Heathcliff. Kirito later exposed Heathcliff as Kayaba’s avatar and defeated him, ending the death game. After escaping Sword Art Online, Kazuto learned Asuna remained comatose in Alfheim Online. Creating a Spriggan avatar, he entered ALO to rescue her, guided by Leafa—his cousin Suguha, unaware of his identity. Their climb to the World Tree’s peak revealed Suguha’s real-world feelings, prompting an in-game reconciliation. Kirito reached Asuna, confronted her captor Sugou, and overpowered him with Kayaba’s residual consciousness, freeing Asuna and other survivors. Later, Kazuto assisted government agent Kikuoka Seijirou in investigating Gun Gale Online, where players died mysteriously under Death Gun’s influence. Using a rare, feminine M9000-series avatar—often mistaken for female—he partnered with player Sinon in the Bullet of Bullets tournament. Kirito deduced Death Gun’s real-world assassination method and defeated him, uncovering ties to SAO’s murder guild Laughing Coffin. This confrontation triggered Kazuto’s PTSD from killing Laughing Coffin members in SAO, a trauma he navigated throughout. Subsequently, Kazuto tested Rath’s Soul Translator for the military Project Alicization. Unaware of the project’s scope, he dived into the Underworld—a simulation built using the World Seed. During testing, Laughing Coffin member Johnny Black assaulted him, causing brain damage. Rath placed him in a prolonged dive to heal, integrating his memories into his Underworld avatar. There, he befriended Eugeo and Alice, trained at the Sword Mastery Academy, and became an elite swordsman. After breaking the Taboo Index, he and Eugeo confronted the ruling Administrator. Eugeo perished in the battle, damaging Kazuto’s fluctlight. He recovered during a Dark Territory invasion, repelling attackers using Eugeo’s residual memories. Kirito’s personality blends social awkwardness with fierce loyalty. His computer-focused upbringing fostered bluntness, yet he protects others selflessly—exemplified by his "Beater" sacrifice. Traumatic losses like the Black Cats’ demise or Asuna’s temporary death trigger emotional volatility. His resolve drives relentless pursuits, such as rescuing Asuna or confronting foes. Physically, his avatars mirror his real-world appearance: black hair, delicate features, and dark attire preferences. In ALO, his Spriggan avatar initially had spiky hair before reverting to his SAO style; in GGO, his rare avatar’s feminine traits caused frequent gender confusion.