Yui

is a mysterious girl found by Kirito and Asuna around the forests of the Floria, the 22nd Floor of the Floating Castle Aincrad.

Her official name is «Yui-MHCP001».

Morning Dew Girl (Aincrad 22nd Floor, October 2024)
Hearing a rumor about a "ghost" that has appeared on the forests around the 22nd floor, Kirito and Asuna decide to check it out themselves. There they meet a young girl who suddenly falls unconscious. They immediately take her back with them and are faced with several concerns about her after finding out that she's not an NPC or part of some random quest. Along with this, she doesn't have a cursor and is far too young to be a normal player as there was an age restriction while setting up the NerveGear at signup, forbidding children below thirteen. Later, during a quest to find the «Army's» leader, Sinker in the dungeon of the «Black Iron Castle», Yui touches a stone left around and immediately regains all her memories. Yui, now knowing her purpose as an AI who will be erased now that the game recognizes her as an alien object, expresses her gratitude to Kirito and Asuna for caring for her. Just as she disappears, Kirito uses his computer skills to retrieve her and store her heart as a tear crystal.

ALfheim Online
After SAO, Kirito unknowingly transfers his SAO data to ALO. Most of his data is deemed unusable and corrupt, but he manages to retrieve Yui from his item window. Here, she is converted into a navigation pixie, and she leads Kirito to the proper locations throughout the land. Yui also finds information stored within the game's databases to help find her mother, Asuna. She is able to return to her regular body, but only does so when alone with Kirito and Asuna.

Gun Gale Online
Yui, through Asuna's cell phone, connects the Bullet of Bullets tournament to the heart beat monitor, letting Asuna and Nurse Aki to see what is currently happening. Seeing that Kirito is losing the fight with his opponent, Yui tells Asuna to hold his hand, which helps Kirito remember of the gun that was still in the holster at his side.

Hardware
Yui is the first version of the «Mental Health - Counselling Program» of «Sword Art Online». She is a «Top-down AI» made to monitor the mental health of the players of SAO. She's a pseudo artificial intelligence built from a program atop an existing computer architecture. Top-down AIs can be considered as a cluster of binary. She later gains access to the real world through a VR probe designed and created by Kirito.

Trivia

 * Yui looks like Sachi. Many fans notice this too, but some take this as a coincidence only.