She is a new player who appears in ALO in volume 7, she leads the Sleeping Knights and quickly becomes known as one of the strongest in the game winning 67 straight matches.

Spoiler

Her guild the Sleeping Knights is a group of terminally ill patients who play using the FullDive Technology MediCuboid, Yuuki's disease is AIDS which she obtained through a bad blood transfer she received as a child due to dystocia, the disease took form when she was bullied in school after it was known she was a carrier; she was then later taken to the hospital where she lived the next 3 years in the new medical FullDive Technology called MediCuboid; it is due to this that her experience in the VR world exceeds even the survivors of SAO, and for this reason even makes her a match for Kirito when he is not Dual wielding. She quickly becomes friends with Asuna due to the reason that she looks similiar to her elder twin sister who died one year prior to her meeting with Asuna.