Yui (ユイ) is a Mental Health Counceling Program AI (handle "Yui-MHCP001") generated by the Cardinal System of the VRMMORPG Sword Art Online to monitor and assist with the players' emotional state as they play. Encountered by Kirito and Asuna on the Floor 22 of Aincrad, she became their companion and adopted AI-daughter with varied level of access to each world's Cardinal System.

Bio

SAO Description (Common)	Childish facial features, long black hair, bright black eyes, short. Default equipment: a white one-piece dress and waist ribbon. During Kirito and Asuna's honeymoon, her ensemble consisted of a long-sleeved pink sweater with purple skirt and red shoes (depicted as brown boots in the anime).
ALO Description	In ALfheim Online, Yui is categorized as a Navigation Pixie, is ten centimeters tall. She wears a long light-pink tube top dress that features detached sleeves and pink anklets She also wears a blue flower in her hair.
Age	(perceived visual) around 12
Birthdate	2022-2024
Gender	Female AI
Family	Kirito, Asuna as in-game foster parents
Affiliation	Cardinal System of the original SAO
VR/AR	SAO, ALO
Occupation	Mental Health Counseling Program in the original SAO
Voice Actors	JP: Itō Kanae; EN: Stephanie Sheh
First Appearance	Novel: Volume 2, Morning Dew Girl, Chapter 1; Manga: Aincrad Manga Stage.003; Anime: Episode 11; Game: Infinity Moment

Personality

Yui has two sides to her: an emotionally involved child that can also read minds (especially pronounced in the beginning as her memory had deteriorated), and an all-out AI control subroutine with a lower-level interface.

In the beginning, when the death game had begun, she became melancholic due to being unable to aid the players. After two years of witnessing suffering and death, she found a couple emanating happiness, and in essence, became so curious that it had overridden the Cardinal System's setting set by Kayaba Akihiko telling her not to interfere. Her memory had become corrupted making her behavior patterns to act like a little child, including enjoying Kirito and Asuna's idea of adopting her, and getting obstinate when she refused to stay behind with Sasha when Kirito and Asuna went to the dungeon on a rescue mission, which saved their lives.

Related Sword Art Online Events

Aincrad Arc

Morning Dew Girl

October 30, 2024: As Asuna and Kirito follow up on the talk of Floor 22 being haunted, Yui wanders aimlessly, and they notice her — a sluggish distant girl in a white dress in the middle of a forest. Kirito notices she has no player cursor over her head and wonders what it means, but then the girl collapses, and Kirito and Asuna take her to their player home nearby and to one of the beds, where she stays unconscious. Asuna thinks the girl could be lonely and joins her in bed.

October 31, 2024: Yui wakes up but cannot remember anything apart from her name. She also has difficulties pronouncing remotely complex words, including the couple's names; they are endeared when Yui decides to make it easier and tries calling them her "papa" and "mama", and accept it. Yui is happy with that.
Yui, Asuna, and Kirito go on a trip to the Town of Beginnings to look for anyone who might know Yui at all. Yui does not find the place familiar at all, including the Teleport Gate plaza that brought about a truly harrowing experience to every player two years ago. She falls asleep on their way to the church where the lost children live. Yui wakes up and reminisces on being along for years in a dark place; then she has a seizure.

November 1, 2024: The vice commander Yulier of the Aincrad Liberation Force requests Kirito and Asuna to find and rescue the entrapped leader of the guild called Thinker from a hidden dungeon beneath the Black Iron Palace. The couple are expecting a trap for themselves and cannot decide either way, so Yui suddenly helps them saying that she knows for sure Yulier is honest, but cannot say how. Yui also refuses to remain behind in safety during the quest, so all four of them go to the dungeon.
In the dungeon's deepest part, Yui touches a dark stone cube in the middle of a safe zone room and suddenly remembers it all. The Fatal Scythe monster appears, and she slashes it in twain with a large flaming sword that turns out to be the GM console's Object Eraser function. So Yui saves the adventurers and remembers she is an AI that was marked for deletion by Cardinal for independent action. Yui thanks Kirito and Asuna for taking care of her, and then disappears. At the last moment, Kirito manages to also use the GM console to save Yui onto his NerveGear's storage as part of his save state, and her 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 full of items with lost pointers, and 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 data bank. From then on, she keeps Kirito company, as well as provides the game tips.

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

Further Canon Stories

Yui keeps enjoying her time with Kirito and the others.

Non-canon

Infinity Moment, Hollow Fragment

Following the boss battle 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) called Strea, who has her own playable avatar. On Floor 100, Strea is imprisoned by the boss, and Yui tries hard to save her. The two endings allow to either record Strea into Yui, or to get a side quest that allows the party to break Strea's avatar out the common way and then transfer her as another Heart item to ALO as well.

Hardware

Yui is the first version of MHCP in SAO — a "top-down AI" (a binary cluster) made to make use of emotions, consider options, and communicate. She is therefore connected to the core hardware and software of SAO and the derived worlds, which lets her appear in those worlds in immersive forms.

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

Trivia

• Yui can access the internet and local networks.
• Yui has an interest in breaking through firewalls.
• Yui is able to hear other player's alarms.
• Yui gets sleepy from Kirito's habit of napping anywhere and everywhere, causing her to hate it.
• Yui enjoys communicating with Pina, as the familiar's intelligence is of a complex structure.
• Yui is able to reproduce her original body in the ALO (likely due to the SAO add-on) but only does it when sleeping together with Kirito and Asuna, as her pixie body is more convenient.
• Annoyed that she is unable to sit on Kirito's spiky hair in ALO, she made him change his revert to his usual hairstyle on June 23, 2025.
• In the fighting game Dengeki Bunko Fighting Climax, Yui runs up to hug Asuna in one of her victory animations.

References

Yui - Fan Wiki