|Spoiler Alert: This article/section contains details about future plotlines described in the light novels, not covered in the anime. Do not proceed unless you want to be spoiled. |
Her official name is «Yui-MHCP001» (Mental Health Counseling Program 001).
She has long, dark hair and bright black eyes. In her most common form, she wears a white one-piece dress. In her Navigation Pixie form, she wears a long light-pink dress with detached sleeves, pink anklets and a blue flower in her hair. In both forms, she goes barefoot.
As Yui was originally a girl without memories, she felt insecure about not knowing who or where her "parents" were. This led her to assume that Kirito and Asuna were her parents. After the couple decided to play along as the girl's parents due to the girl's psychological condition, she became cheerful and shows great affection for her so-called "parents". According to Asuna, her behavior was like that of a baby despite her child-like appearance. However, she lacked any sense of fear, regardless of the intensity of the danger she might be in and even showed signs of rebellion and recklessness when Kirito told her that she had to stay with Sasha when he and Asuna wanted to go to a dungeon to rescue Thinker.
After she regained her memories, she told her in-game parents that she 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. After nearly 2 years of only seeing insanity, misery, and anger, she finally noticed a couple, Kirito and Asuna, who showed happiness and joy and, even though she was not allowed to interact with players, she continued monitoring them and even came to the forest near their home to be close to the two to experience their emotions. After being comforted by Kirito and Asuna, she regained her joy. Even though she was about to be deleted by the Cardinal System, Yui showed no sign of repentance, but tears of happiness, for she was able to be with her in-game parents.
Upon being reactivated in ALfheim Online she still displayed a deep attachment to Kirito and Asuna, continuing to act as a surrogate daughter. She also scolded Kirito when he received affection from other women aside from Asuna, stating her "papa" is "cheating", but she did, however, accept Leafa and later Sinon into Kirito's life.
After hearing about a rumored "ghost" that had appeared on the forests around the 22nd Floor, Kirito and Asuna decided to check it out themselves. There they met a young girl who suddenly fell unconscious. Initially, Asuna thought that the girl was the ghost and was scared, but Kirito rushed to Yui without hesitation. They immediately took her back to their Log House with them and were faced with several concerns about her. They deduced that she was not an NPC, as she did not have a quest flag, and that picking her up did not result in a NPC harassment warning. She also lacked a player cursor, which every player had, and was far too young to be a normal player as setting up the NerveGear signup forbade children below thirteen years old. Later, during a quest to find «The Army's» leader, Thinker, in the dungeon of the «Black Iron Palace», Yui touched a stone cube at the center of the safe zone at the deepest part of the dungeon and immediately regained all her memories. She proceeded to defeat The Fatal Scythe using the GM console's «Object Eraser» function that materialized as a giant flaming sword.
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 her and store her core programming as an in-game item, shaped like a tear crystal. The item was named «Yui's Heart» and Yui was saved as environmental data on Kirito's NerveGear.
After SAO, Kirito unknowingly transferred his SAO data to ALO. Most of his data was deemed unusable and corrupt, but he managed to retrieve Yui from his item window (she was listed as an item named «MHCP001» (Mental Health - Counseling Program) in the item window).
Here, she is classified as a Navigation Pixie by the system, and she leads Kirito to the proper locations throughout the land. Yui also provided information, mainly through the provision of maps stored within the game's database, along as a player-detector and giving advice when needed to help Kirito find Asuna. She is still able to go back to her regular body, but only does so when alone with Kirito and Asuna.
With Yui's help Kirito was eventually able to unite and rescue Asuna from Sugou Nobuyuki (Oberon). When they met Asuna again, Yui returned to her regular body, and they shared a "family hug". Unfortunately, before they could escape, Sugou intervened and attempted to delete Yui, but she managed to escape to Kirito's and Asuna's relief. Yui attempted to reunite with Asuna and Kirito afterwards, but she was only able to after Kirito successfully destroyed Sugou's avatar and helped Asuna log out of the game.
While Yui was sad that Asuna had already left, she understood why Asuna had to return to the real world. Kirito promised Yui that both he and Asuna would visit her soon. Before Kirito left to meet Asuna in the real world, Yui informed Kirito that a rather large file («The Seed») 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.
Sometime later, Yui joined Kirito, Asuna, and their friends on a new adventure in ALO.
Prior to Kirito entering GGO, she 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 of the gun that was still in the holster at his side.
After Kirito was critically injured by a former Laughing Coffin member, Johnny Black, and subsequently kidnapped by Kikuoka Seijirou, 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.
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 Kirito.
Her core program runs and is stored on Kirito's NerveGear local memory, and is about 3TB large.
- 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.
- So far, the only girls in the entire series that Yui likes that are around Kirito (besides Asuna), are Suguha and Shino.
- 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 appears in one of Asuna's winposes in Dengeki Bunko Fighting Climax where she runs up to Asuna and hugs her.
- At the end of the web novels Yui evolves into a Bottom-Up AI.