Asuna hasn’t been able to find Yuuki since they visited the monument. Thanks to Kirito, she’s able to find her again in the real world.
Asuna has been trying to contact Yuuki in the game, but isn’t getting a response. She’s visited by Siune who tells her that she won’t see any of them again and that it’s for her own good. Asuna is dejected and comes to school in a bit of a funk. She goes to see Kirito on the roof where he’s still working on his tech to allow Yui to see into the real world. He gives her a lead that sends her to a hospital. It is a bit surprising that he managed to get this information. I guess it makes a little sense that he would know where to look since, as Yuuki said, he figured out her secret. It tuns out that Yuuki was using a device called a Medicuboid. It was a relative of the Nerve Gear and Amusphere devices that are used for the full dive VRMMO games. Asuna meets with Yuuki’s doctor at the hospital and tells her about the device and how it works. He also informs her that the device is usually used as a form of anesthesia for terminally ill patients. He cautions Asuna that she may not want to learn the full truth of Yuuki’s situation.
It turns out that Yuuki is in fact a terminally ill child. She was born through C-section and, during the procedure was given a bad blood. As a result she contracted AIDS from birth. I have to admit that I think this is the first time I’ve heard of AIDS being in an anime. It’s definitely the first time I’ve seen it in a shounen show. They had medicated Yuuki, but her immune system failed. In exchange for putting her in a “clean room” her parents agreed to let her be a test subject for the Medicuboid. As her condition worsened, she spent more and more time in the virtual world in order to escape the pain her body was in. Upon hearing this, Asuna realized why Kirito said that she was “a product of the full dive environment”. The doctor also tells Asuna that Kirito did have a sister, who was a bit like Asuna. All of this is taking place while Yuuki si watching from inside the virtual world.
She tells Asuna that she can meet her by using the full dive device in a nearby room. She goes back to the same tree where Asuna first dueled with her. Yuuki admits the truth about the Sleeping Knights. The guild is composed of other terminally ill patients from different places. Several members of the guild (including Yuuki’s sister) had already died and they had agreed that they would disband when the other member’s time came. As it is, two other members of the guild that have been told they only have three months to live. Asuna insists that Yuuki must have other things that she wanted to accomplish. Yuuki says that she is satisfied, but mentions that she did want to go to school. As the two look off into the sunset, Asuna mentions that it may be possible for her to go to school.