Kazuto Kirigaya

The main protagonist throughout the main story, excluding side stories. Born on October 7, 2008, in Kawagoe City, Saitama Prefecture. According to a special bookmark from Dengeki Bunko, he is 163 cm tall. He has a well-proportioned face with androgynous features, which he personally dislikes. He lost his parents in an accident shortly after birth and was adopted by the Kirigaya family, his mother's sister and her husband. After accidentally learning about his background at the age of 10, he grew into a boy who feared interacting with others and was poor at socializing, seeking mental peace in the virtual world. At 14, he got caught up in the SAO incident and grew mentally through various incidents related to the virtual world. He was ranked first in the male character category of "This Light Novel is Amazing!" (Takarajimasha) in 2012, 2013, and 2018.

Due to his personality, described by Asuna as "spreading his arms to the limit to accept everything and never throwing it away," he is trusted by many characters in the story and often receives affection from women, though he is devotedly in love with Asuna and shares a strong bond with her. He faced the deaths of members of the "Moonlit Black Cats," including Sachi, during the SAO incident, and his trauma led him to try to bear everything alone. He lost many comrades in the same incident and, although reluctantly, killed criminal players, leaving him with emotional scars that have not healed even after returning from UW. He still cannot check on the well-being of many SAO survivors he knows. Losing his same-sex partner and best friend Eugeo in UW made him believe he could never have a friend of the same gender again, but he decided what he should do based on his experiences in SAO and UW and resolved to enter the Tokyo Institute of Technology, Kayaba's alma mater, to make the virtual world useful in the real world.

As a VRMMO player, he has high virtual world aptitude and exceptional skills. In combat, he uses a one-handed sword without a shield, favoring physical attacks with heavy strikes and employing various sword skills. In SAO, he was chosen as the holder of the unique skill "Dual Blades" and devised numerous out-of-system skills in SAO and the new ALO. He generally seals "Dual Blades," only unlocking it when the game is no longer just a game, even after the safety issues were resolved in the old ALO. In GGO, he performs feats like cutting bullets from their prediction lines and reading opponents' sniping points from their gaze. From his SAO experience, he takes a strict stance against PK motivated by personal gain, even in play.

His avatars are consistently equipped with black-themed armor, avoiding metal armor that hinders movement. In the old ALO, his race was Spriggan, and he wielded a greatsword with a reversed hairstyle, but in the new ALO, he reset his character data and soon let his hair down at Yui's request. In GGO, he uses a rare avatar that looks like a long-haired woman, wielding an unconventional dual-wielding style with a photon sword and an automatic pistol. In UW, he wields the "Night Sky Sword," made from the branch of the giant Gigas Cedar, as his exclusive weapon. In UR, unlike before, he equips metal armor and a longsword and tames a black panther-type monster.

According to the author, Reki Kawahara, there is no specific model for him.