Yui

Yui (ユイ) is an Artificial Intelligence (AI), found by Kirito and Asuna around the forests of the 22nd Floor of the floating castle Aincrad. Her official name is «Yui-MHCP001» (Mental Health Counseling Program 001).

Sword Art Online
Yui had long, dark hair and bright black eyes. Her default equipment consisted of a one-piece white dress with a ribbon tied around her waist. She temporarily wore a long-sleeved pink sweater and a purple skirt, as well as red shoes (brown boots in the anime), during her time with Kirito and Asuna.

ALfheim Online
As a Navigation Pixie, Yui is ten centimeters-tall and wears a long, light-pink tube top dress with detached sleeves, pink anklets, and a blue flower in her hair.

Personality
Yui had become melancholic when the death game began, as she was not allowed to comfort players and was forced to continue monitoring their psychological parameters. When she finally, after nearly two years of only seeing insanity, misery, and anger, noticed a couple, expressing happiness and joy, she became overwhelmed with curiosity, which led to her ignoring the Cardinal System's command not to interact with players.

As her memory became corrupted, Yui's behaviour regressed to that of a child. She became cheerful when Asuna and Kirito pretended to be her parents, but she also showed signs of obstination, such as refusing to stay with Sasha when Asuna and Kirito decided to go to a dungeon to rescue Thinker.

Morning Dew Girl
On October 30, 2024, while Asuna and Kirito were investigating rumors of a ghost in the forests on the 22nd Floor, Yui wandered close to the couple. When the pair noticed her presence, the young girl silently watched them for several seconds, before collapsing. Out of concern for her well-being, Yui was taken by Asuna and Kirito to their home and settled in their bed, where she remained unconscious for the remainder of the day.

At 07:50 the next morning on October 31, Yui began humming to Asuna's wake-up alarm while half-awake. After being fully awakened by the couple, she shared her name with them but could not remember anything else. As the little girl experienced difficulty pronouncing their names during breakfast, she was encouraged to simplify their names. As such, Yui hesitantly decided to address them as her parents, becoming overjoyed when the startled duo accepted the roles.

After lunch, Yui was asked to join Asuna and Kirito in looking for any acquaintances of hers. To prepare for the trip, Yui followed the couple's instructions to allow them to change her equipment to more appropriate attire for the season. Once dressed in warmer clothing, she happily accompanied them outside the house.

When the trio arrived at the Town of Beginnings, Yui was unable to recognize anything familiar from the Teleport Gate Plaza, falling asleep as Asuna and Kirito carried her to a church where other young lost children were living. When she awoke, the girl extended her hand into space and quietly recalled how she used to always be alone in a deep, dark place, before temporarily losing consciousness after suffering an unusual seizure.

During breakfast the next morning on November 1, the trio received a request from Yulier, the vice commander of the Aincrad Liberation Force, to rescue the guild's leader Thinker from a hidden dungeon beneath the Black Iron Palace. When Asuna and Kirito expressed hesitance to help the woman without first verifying the authenticity of the story behind her entreaty, Yui affirmed that Yulier was being honest, although she was unsure why she was so certain.

Having refused to remain behind with Sasha, the player overseeing the other children, Yui accompanied Asuna, Kirito, and Yulier as they traversed the Hidden Dungeon, cheerfully praising the others during their battles against the dungeon's monsters. After two hours of traveling and fighting, they finally reached the safe zone where Thinker was located. However, when Yulier accidentally triggered the appearance of The Fatal Scythe, the little girl was taken by the woman to the safe haven to be teleported to safety via a crystal while Asuna and Kirito distracted the boss.

As Yui entered the safe zone, though, she touched the GM command console disguised as a black stone cube pedestal in the center of the room, regaining her memories. Having accessed the system through the terminal, the girl left the safe haven while Thinker and Yulier were teleporting away and hurried to Asuna and Kirito, who had been incapacitated by The Fatal Scythe, repelling the boss's attack on her with an automatic Immortal Object shield before defeating it with an Object Eraser materialized as a giant flaming greatsword.



Afterwards, Yui, now knowing her purpose as an AI who was going be erased since the system recognized her as an "alien" due to having disobeyed the system's orders, expressed her gratitude to Kirito and Asuna for taking care of her. Just as she disappeared, Kirito used the still active admin access and his computer skills to retrieve and save her as environmental data on his NerveGear and store her core programming as an in-game tear crystal-shaped item named «Yui's Heart».

Fairy Dance Arc


Yui was revived in ALfheim Online as a Navigation Pixie after Kirito found Yui's data imported from Sword Art Online. Yui led Kirito to the proper locations throughout the land. Yui also provided information, mainly through the provision of maps stored in the game's database, along as a player-detector and giving advice when needed to help Kirito find Asuna.

Yui and Kirito were eventually able to rescue Asuna from Oberon (Sugou Nobuyuki). During their reunion, Yui returned to her regular body, and they shared a hug. However, their reunion was interrupted by Oberon. As Yui was in danger of being deleted, she fled the scene. Yui attempted to reunite with Asuna and Kirito afterwards, but she was only able to after Oberon was defeated and Asuna was logged out.

While Yui was sad that Asuna had already left, she understood why Asuna had to return to the real world. Before Kirito left to meet Asuna in the real world, Yui informed Kirito that a rather a large file had been transferred to his NerveGear's storage. Yui also informed Kirito that regardless of what happened to ALO, she would be fine because she existed in Kirito's NerveGear.

Some time later, Yui joined Kirito, Asuna, and their friends on a new adventure in ALO.

Celeste Fairy
When summoned by Kazuto during his ride on his Bianchi road bicycle, Yui appeared to him via a display in his sunglasses. After enjoying the sight of cherry blossoms through a camera, Yui was requested to help Kazuto with the navigation and data logging. After twenty minutes of Kazuto's cycling, Yui noticed a transmission coming from his bicycle. At his request, Yui paired with the device emitting the signal and visualized a girl, named Cel, residing in it. As Cel only reacted to input from a monitor, Yui offered to convey his voice commands to her. Before Cel was returned to her creator, Yui conversed with her and receive some information during their time together.

Rainbow Bridge
At the end of the «Deep Sea Plunderers» quest, the seven-member party of Agil, Asuna, Klein, Kirito, Leafa, Lisbeth, and Silica, along with Yui and Pina were brought back to Alfheim by a whale. After the group disembarked the whale at a beach in Sylph territory, Yui thanked the whale for letting the group ride on its back. Due to the group's concern about the conversation between Leviathan and Kraken, as well as the lack of a way to review the dialogue of NPCs, Yui reproduced their conversation for the group.

Later, Yui was also present during the group's attempt to reach the other side of the «Lightning Dragon's Nest».

Phantom Bullet Arc
Prior to Kirito entering Gun Gale Online (GGO), Yui was mentioned in a conversation between Asuna and Kirito about the possibility of her having a "younger sister" in the future and how great it would be if Yui could cross over into the real world.

Later, during Kirito's final battle with Sterben (Death Gun), Yui connected the Bullet of Bullets tournament broadcast to the heart beat monitor via Asuna's cell phone, which allowed Asuna and Nurse Aki see what was currently happening. After seeing that Kirito was losing the fight with his opponent, Yui told Asuna to hold his hand, which helped Kirito remember the gun that was still in the holster at his left side.

Ceramic Heart
While wandering a field in ALfheim Online, Yui wondered about the meaning of love, an emotion she could not understand. At the same location ten days before, she had met a boy who confessed to her. Thinking that he wanted to monopolize her, she had told him that she refused to belong to him. However, to her surprise, she learnt that this was not what the boy was seeking with his confession, as he claimed his intention was just to finally speak to her before resigning from the game. When Yui came to Asuna to share her anxiety due to not understanding the boy's words, she received a suggestion ask the boy in person.

However, the boy had already deleted his account, thus Yui hacked the game's server, discovering the boy's place of residence. Yui was taken to the house by Asuna through an Audiovisual Bidirectional Communication Probe. There, she learnt that the boy was actually a recently deceased elderly man who had a hobby of going into VRMMO games in his last years. She also discovered that before the elderly man passed away, he had told his adult daughter that he had seemingly met his deceased wife in ALO.

In the present, based on her experience, Yui assessed that the feeling of loss was what it truly meant "to love".

Alicization Running
After Kirigaya Kazuto was critically injured by a former Laughing Coffin member, Kanamoto Atsushi (Johnny Black), and subsequently kidnapped by Kikuoka Seijirou, Yuuki Asuna and her friends began to think there was no hope in locating Kirito. However, Yui convinced her mother not to give up, just like when Asuna was trapped by Sugou, Kirito refused to give up on finding and rescuing Asuna. It was enough to restore Asuna's resolve and with Yui's help, along with Koujiro Rinko (the deceased Kayaba Akihiko's love interest), Asuna was eventually successful in finding Kirito.

Infinity Moment/Hollow Fragment
After the 75th Floor boss battle, Kirito and Asuna noticed that Yui's Heart was glowing upon reaching the 76th Floor. Upon opening the item, Yui was restored and became a partner character for Kirito.

During the events of the Hollow Fragment storyline, she helped Kirito understand the error that befell Philia upon arriving in the Hollow Area and she managed to recover some of her admin credentials during an event on the 76th Floor.

In the game, she has a younger sister known as Strea, another Mental Health Counselling Program like her, except that Strea has the power to play the game and fight the boss monsters alongside the Clearing group. When Strea was overtaken by the 100th Floor boss, Yui did all that she could to save her. In one ending, she managed to save Strea's life by absorbing her into herself, telling Kirito afterwards that she was alive. In the other ending, only accessible by completing a side quest in the Hollow Area, Yui and Kirito created an "Advance Difficulty Test" in hopes of freeing Strea. While it did not go as first hoped, they managed to free Strea from her prison, and Strea later had the option of becoming a game item, allowing both girls to go to ALfheim Online with Kirito.

Hardware
Yui is the first version of the «Mental Health - Counseling Program» of «Sword Art Online». She is a «Top-down AI» made to monitor the mental health of the players of SAO. She was made to be able to mimic emotions and be able to interact with the players of SAO. During the events of SAO she becomes self-aware and a fully realized artificial intelligence built from a program atop an existing computer architecture, which is why she usually appears in programs based on SAO. Top-down AIs can be considered as a cluster of binary.



She later gains access to the real world through a VR probe (Audiovisual Bidirectional Communication Probe) designed and created by Kirigaya Kazuto.

Her core program runs and is stored on Kazuto's NerveGear local memory, and is about 3TB large.

Gallery

 * See:Yui/Image Gallery

Trivia

 * The original way Yui was supposed to monitor players can be deduced from the opening animation.
 * Yui is also capable of accessing search engines and computer networks. Her thought processes are extremely effective, having an extraction rate of 95% of useful data.
 * Yui thinks that Kirito's most annoying habit is him napping anywhere and everywhere, as she gets sleepy from them too.
 * Yui enjoys communicating with Pina, as the familiar's intelligence is of a complex structure.
 * Although Yui would like a younger brother or sister, so that she would have someone to talk to, she also wishes to monopolise her Papa and Mama.
 * Yui is able to reproduce her original body in ALO, but only uses it when sleeping together with Kirito and Asuna, as her pixie body is more comfortable in many ways.
 * Yui is able to hear other player's alarms.
 * Yui's interest is breaking through firewalls.
 * Yui disliked Kirito's spiky hair in ALO, as it was hard to sit on, thus on June 23, 2025, she had him change his hairstyle to his usual one.
 * In the non-canon game adaptation, Hollow Fragment, Yui displays her sewing skills by making swimwear for herself, Asuna and Kirito.
 * Yui appears in one of Asuna's win poses in Dengeki Bunko Fighting Climax where she runs up to Asuna and hugs her.

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Yui Yui Yui pt-br:Yui Юи Yui