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