While Asuna, Leafa, Silica and Lisbeth were doing their homework in Kirito and Asuna's Log House on the 22nd Floor of New Aincrad, Lisbeth informed Asuna of a player known as "Zekken" (Absolute Sword) who was challenging players on the 24th Floor, while offering an 11-hit Original Sword Skill as a reward. Asuna was told that the mysterious player had actually converted from a different game and that over thirty players who had attempted to challenge the player, including Leafa and even Kirito, were defeated. Hearing that Kirito had said something to the player before being defeated and not sharing what he had said with the others and thinking that the mysterious player had come for a reason, Asuna decided to challenge the player as well. To her surprise, the player was actually a girl named Yuuki.
(Source: SAO Wikia)