Kirito, formally Kazuto Kirigaya, is a central figure shaped by extensive experiences across multiple virtual realities. Orphaned young and adopted by his aunt and uncle, early computer exposure forged his technical aptitude and drew him into virtual worlds.

Trapped within the deadly Sword Art Online death game, he operated under the avatar name Kirito. This period, marked by the loss of companions like Sachi, deeply scarred him, fostering survivor's guilt and emotional guardedness despite an innate drive to protect others. His combat skill and signature black attire earned him the moniker "Black Swordsman."

Later, during Project Alicization, he unknowingly participated in a military experiment testing the Soul Translator. Inside the simulated Underworld, his fluctlight merged with the population, living a fabricated childhood alongside Eugeo and Alice Zuberg. A pivotal event saw the trio near the Dark Territory; Alice crossed the forbidden boundary and was abducted by Integrity Knights. Kirito resisted her capture while Eugeo froze, an incident haunting both. Post-experiment, his Underworld memories were blocked.

After real-world poisoning by Johnny Black, severe brain damage necessitated a medically induced coma and reconnection to the Soul Translator aboard the Ocean Turtle. This forced return to Underworld initially stripped him of real-world memories. He encountered Eugeo, now a woodcutter tending the Demon Tree, both devoid of shared past recollection. Kirito gradually regained combat abilities, aiding Eugeo in overcoming trauma to fell the Demon Tree before journeying to Central City to rescue Alice.

In Underworld, Kirito trained at the Sword Mastery Academy, rising to elite status and adopting a black uniform. His actions and bond with Eugeo—culminating in Eugeo's sacrifice against Administrator—triggered profound emotional turmoil. Following Eugeo's death and Administrator's defeat, an external attack on the Ocean Turtle during his real-world link caused catastrophic feedback, rendering his Underworld avatar comatose. This vegetative state persisted through much of the ensuing War of Underworld. Residing in a secluded Rulid Village cabin under the care of the exiled Integrity Knight Alice, his inert presence became a psychological anchor for allies like Alice, Ronie, and Tiese, motivating their defense of the Human Empire against invading Dark Territory forces and external threats like Gabriel Miller.

His eventual recovery involved confronting internalized visions of past failures and losses. This culminated in a resurgence of will, empowered by Underworld's "Incarnation" system where mental fortitude shapes reality. Reawakened, he rejoined the battle wielding the Night-Sky Sword. In the final confrontation against Gabriel Miller, manifested as void darkness, Kirito fought to protect Alice and Asuna as they raced towards the World's End Altar for fluctlight extraction, a battle with immense temporal stakes due to accelerating time differential threatening to strand him for centuries.

Core traits define him: exceptional swordsmanship, strategic adaptability in virtual combat, deep loyalty to companions, and a tendency toward self-sacrifice. Recurring psychological struggles involve reconciling past traumas and protecting those close to him, often driving pivotal decisions. His influence extends beyond personal actions, inspiring collective resistance among allies across virtual worlds.