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 met a young girl who suddenly falls unconscious. They immediately took her back with them and was faced with several concerns about her after finding out that she's not part of a random quest or an NPC. She apparently doesn't have a cursor and was far too young as there was an age restriction while setting up the NerveGear, before being able to sign up, forbidding children below thirteen. Later, at a quest to find the «Army's» leader, Sinker in the dungeon at the «Black Iron Castle». Yui touches a stone left around, and immediately knew why she was there and all her memories before it was damaged. Yui, now knowing her purpose as an AI and will be erased, as she became an alien object, expresses her gratitude to Kirito and Asuna for caring her. Just as she disappears, Kirito uses his computer skills to retrieve her and store her heart as a tear crystal. Yui was in a lot more arcs after Morning Dew Girl. It's just small information that has been given later on.

ALfheim Online
After SAO, Kirito unknowingly transferred his SAO data to ALO. Most of his data was deemed unusable, but he managed 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 databases to help find her mother, Asuna.

Gun Gale Online
Yui, through Asuna's cell phone, connects the Bullet of Bullets contest 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.