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».

Chronology
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.

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.