Yui (SAO)

, officially named «Yui-MHCP001» (Mental Health Counseling Program 001), is a supporting character throughout the Sword Art Online series.

Background
Yui is a top-down Artificial Intelligence (AI) program that was originally developed by Argus to address player mental health concerns in Sword Art Online (SAO). However, when Kayaba Akihiko turned Sword Art Online into a death game, Yui was forbidden from interfering with players whatsoever by the Cardinal System, which led to her collapsing due to being forced to monitor player mental health without being able to fulfil her duties. Found in this broken state by Asuna and Kirito around the forests of the 22nd Floor of the floating castle Aincrad, she was mistaken for a lost child player and adopted by the couple. After recalling her actual identity in a Hidden Dungeon on the 1st Floor, Yui ignored the Cardinal System's orders and exercised her Game Master rights to save Asuna and Kirito from The Fatal Scythe, which led to her being flagged for deletion by the system. Before this could happen, Yui's program files were separated from the system, converted into an in-game object, and stored in Kirigaya Kazuto's (Kirito's) NerveGear. After Sword Art Online was cleared, Yui's data was imported into ALfheim Online along with Kirito's account due to the game being based on the data from Sword Art Online's servers. In ALfheim Online, Yui is treated as a Navigation Pixie by the game's system, granting her access to the game's database. After the Fairy Dance Arc, Yui's data was moved outside of Kazuto's NerveGear, granting her access to devices outside of games.

Appearance
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.

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 hidden dungeon to rescue Thinker.

Trivia

 * Yui is voiced by Kanae Itou in the anime adaptation of Sword Art Online.
 * She shares a voice actor with: Suzuno Kamazuki (The Devil is a Part-Timer!), Ringo Akai (Ookami-san Series), Junko Oogami (Shakugan no Shana S), Aoi Ogiyama (Ro-Kyu-Bu!), Kanase Kanon (Strike the Blood), Sakura Toriumi (Angel's 3Piece!) and Ruiko Saten (A Certain Scientific Railgun).