Yui (ユイ) is a Mental Health Counseling Program AI (nickname "Yui-MHCP001") generated by the Cardinal System in the VRMMORPG Sword Art Online to monitor and help manage players' emotional states while they play. Encountered by Kirito and Asuna on the 22nd floor of Aincrad, she became their companion and adopted AI daughter with varying levels of access to each world's Cardinal System.

Biography

Personality

Yui has two sides: an emotionally involved child who can also read minds (especially pronounced early on as her memory had deteriorated), and an AI control subroutine with a lower-level interface.

Initially, when the killing game began, she became melancholic due to her inability to help the players. After two years of witnessing suffering and death, she found a couple emanating happiness, and in essence, became so curious that she overrode the Cardinal System setting set by Kayaba Akihiko telling her not to interfere. Her memory became corrupted, causing her to act like a small child, including liking Kirito and Asuna's idea of ​​adopting her, and becoming stubborn when she refused to stay with Sasha when Kirito and Asuna went to the dungeon on a rescue mission, which saved their lives.

Related Sword Art Online Events

Aincrad Arch

Daughter of the Morning Dew

October 30, 2024 : As Asuna and Kirito continue to talk about the haunted Floor 22, Yui wanders aimlessly and they notice her—a distant, lazy girl in a white dress in the middle of a forest. Kirito notices she doesn't have a player cursor over her head and wonders what that means, but the girl collapses and Kirito and Asuna take her to their nearby player house and one of the beds, where she remains unconscious. Asuna thinks the girl might be alone and joins her in bed.

October 31, 2024 : Yui wakes up but remembers nothing but her name. She also has difficulty pronouncing somewhat complex words, including the couple's names; they become very attached when Yui decides to make it easier for them and tries calling them her "daddy" and "mommy," and accepts it. Yui is happy about this.
Yui, Asuna, and Kirito set off on a journey to the City of Beginnings to look for someone who might know Yui. Yui finds the place completely unfamiliar, including the location of the teleportation gate that caused a truly distressing experience for all the players two years ago. She falls asleep on the way to the church where the lost children live. Yui wakes up and remembers spending years in a dark place; then she has a seizure.

November 1, 2024 : Vice-Commander Yulier of the Aincrad Liberation Force asks Kirito and Asuna to find and rescue the guild leader, Thinker, trapped in a hidden dungeon beneath the Black Iron Palace. The pair expect a trap and cannot decide one way or the other, so Yui suddenly helps them by saying that she knows for sure that Yulier is honest, but cannot say how. Yui also refuses to stay safe during the quest, so the four of them head into the dungeon.
In the deepest part of the dungeon, Yui touches a dark stone cube in the middle of a safe zone room and suddenly remembers everything. The Fatal Scythe monster appears and she slices it in half with a large flaming sword that turns out to be the Object Eraser function on the GM console. Yui thus saves the adventurers and remembers that she is an AI that was marked for deletion by Cardinal for independent action. Yui thanks Kirito and Asuna for taking care of her, then disappears. At the last moment, Kirito also manages to use the GM console to save Yui to his NerveGear's storage as part of his save state, and his program manifests as an in-game item "Yui's Heart" assigned to his inventory.

Fairy Dance Arc

Kirito transfers his character from SAO to ALO , logs in, and finds his inventory filled with items with lost pointers, as well as Yui's heart. He activates Yui's heart, and Yui is brought back to life as a Navigation Pixie connected to the game's help databank. From then on, she keeps Kirito company and provides him with gameplay advice.

Eventually, they find Asuna, who has been imprisoned by Oberon, and with the help of Yui and Kayaba Akihiko, Kirito manages to rescue her. Yui sends the two to the real world and records her own state onto Kirito's NerveGear, thus perpetuating her existence.

Other canonical stories

Yui continues to enjoy her time with Kirito and the others.

Non-canon

Moment of Infinity None Hollow Fragment

After the boss fight on Floor 75, Asuna and Kirito notice Yui's heart glowing on Floor 76. They activate the item and Yui is revived, becoming a partner character for Kirito.

She has a younger sister (another MHCP AI) named Strea, who has her own playable avatar. On the 100th floor, Strea is imprisoned by the boss, and Yui does her best to save her. Both endings either save Strea to Yui, or grant a side quest that allows the party to release Strea's avatar the usual way, then transfer her as another ALO Heart item as well.

Material

Yui is the first version of MHCP in SAO —a "top-down AI" (a binary cluster) designed to harness emotions, consider options, and communicate. She is therefore connected to the core hardware and software of SAO and its spin-off worlds, allowing her to appear in those worlds in immersive forms.

Later, Kirigaya Kazuto builds a feedback device that allows Yui to interact with the real world.

Anecdotes

• Yui can access the Internet and local networks.
• Yui better break through the firewalls.
• Yui is able to hear other players' alarms.
• Yui falls asleep because of Kirito's habit of napping anywhere and anytime, which makes her hate it.
• Yui enjoys communicating with Pina, as her familiar's intelligence is of a complex structure.
• Yui is able to replicate her original body in ALO (probably due to the SAO add-on) but only does so when sleeping with Kirito and Asuna, as her pixie body is more practical.
• Annoyed that she couldn't sit on Kirito ALO 's spiky hair, she made him change his hairstyle back to his usual hairstyle on June 23, 2025.
• In the fighting game Dengeki Bunko's Climax, Yui runs to hug Asuna in one of her victory animations.

References

Yui - Fan Wiki