Yui

is a mysterious girl found by Kirito and Asuna around the forests of the 22nd floor.

Background
Hearing a rumor about a "ghost" that has appeared on the forests around the 22nd floor, Kirito and Asuna decided to check it out themselves. There they found a young girl clothed in a white one piece dress and was suprised because she suddenly collapsed and lost consciousness.

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.