As the players progress through the game Kirito eventually befriends a young girl named Asuna Yuuki, forming a relationship with and later marrying her in-game.

After the duo discover the identity of Kayaba's secret ID, who was playing as the leader of the guild Asuna joined in, they confront and destroy him, freeing themselves and the other players from the game.

Several hours later, he is informed by Agil, another SAO survivor, that a figure similar to Asuna was spotted on "The World Tree" in another VRMMORPG cyberspace called Alfheim Online (ALO).

Assisted in-game by his cousin Suguha "Leafa" Kirigaya and Yui, a navigation pixie (originally an AI from SAO), he quickly learns that the trapped players in ALO are part of a plan conceived by Sugou to perform illegal experiments on their minds.

The goal is to create the perfect mind-control for financial gain and to subjugate Asuna, whom he intends to marry in the real world, to assume control of her family's corporation.

Before leaving ALO to see Asuna, Kayaba, who has uploaded his mind to the Internet using an experimental and destructively high-power version of NerveGear at the cost of his life, entrusts Kirito with The Seed – a package program designed to create virtual worlds.

Kazuto eventually reunites with Asuna in the real world and The Seed is released onto the Internet, reviving Aincrad as other VRMMORPGs begin to thrive.

[6][7] Kirito makes a cameo appearance in the eighth episode of the anime series Eromanga Sensei, with Yoshitsugu Matsuoka reprising his role.

"[9] However, she praised the positive aspects of Kirito's character, and his "competent and noble" self distinguishes him from other harem protagonists in that he actually earns the girls' affection.

[9] Carly Smith of The Escapist criticized Sword Art Online II and compared it unfavourably to the original storyline, summarizing "welcome to Sword Art Online II: the show where everyone loves Kirito and nothing matters", and that the anime doesn't "start to shine" until "Kirito is pushed to the sidelines".

[15] Asuna and Kirito were awarded first and second respectively in a character poll by Dengeki Bunko (the publisher of Sword Art Online) for their light novels.

[19] "Kazuto" (Kirito) was also fourth on a list of the "top 20 anime characters Japanese fans would name their children after"; "Asuna" placed first.

[20] Italian restaurant La Ricetta in Zama, Kanagawa features pancakes prepared with anime and video game character art, including Kirito and Asuna.

[22] Fan-made replicas of Asuna's "Lambent Light" and Kirito's "Elucidator" rapiers have been built by the Man At Arms team of blacksmiths and craftsmen.

"[25] A series of anime-inspired Christmas cards featured characters such as Rin Matsuoka from Free!, Godoka from Puella Magi Madoka Magica, and Kirito.