Yui

Yui's History

Background

Yui was originally created by the developers of the Cardinal System to attend to Sword Art Online players suffering from severe mental health issues. When the game officially launched on November 6, 2022, however, she was ordered by the Cardinal System not to interfere with any of the players. Trapped between her directive to address player mental health concerns and her inability to interact with them, Yui could only monitor their mental health decline, gradually collapsing over the errors resulting from the contradiction.

One day, in the middle of her usual monitoring, Yui detected two players whose mental parameters were significantly different from the norm. Her curiosity aroused by the joy and peace she observed from the duo, Yui eventually developed a desire to become closer to them. To that end, Yui manifested in a physical form at the nearest system console to their home in search of them.

Aincrad Arc

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.

Now aware of her identity as an AI and that she was going be erased due to having disobeyed the system's orders, Yui expressed her gratitude to Asuna and Kirito for their care. 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

On January 20, 2025, Yui was revived in ALfheim Online as a Navigation Pixie after Kirito found Yui's data imported from Sword Art Online. Yui supported Kirito on his journey to find Asuna by providing information from the game's database, as well as detecting players.

Yui and Kirito were eventually able to reach Asuna at the World Tree. During their reunion, Yui returned to her regular body, and they shared a hug. However, their reunion was interrupted by Oberon (Sugou Nobuyuki). 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.

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.

⚠️ There is missing information between these two events ⚠️

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 Arc

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.

⚠️ There is missing information after this event ⚠️