I had always thought that it was Sugu, Kazuto's sister. That, somehow, Kirito briefly glimpsed the real world. But, in retrospect, it was Asuna.
Firstly, it cannot be Sachi: Sachi is dead, and other than the strange occurrence of Griselda's brief appearance, the dead do not reappear. If Asuna hadn't seen had Griselda, too, then I would be apt to think that this was a hallucination. For now I will call it an anomaly or glitch in the system. I am sure there could be many considering Yui, Asuna defying paralysis, and Kayuba's surprise when Kirito refused to die before issuing the fatal blow, just to name a few.
As I said above, the first time I saw the episode and the black-haired girl, I was certain it was Sugu. I thought that Kayuba had lied and you didn't really die when your HP hit zero, he had just said that to give a more realistic and final feel to the game so that the players would take it more seriously. After Kirito finished it and Kayuba confirmed that everyone who had died in-game was really dead, episode 9 left me confused. How had Kirito then been able to see his sister?
Had Kirito, being so near to death, briefly viewed his sister crying over him in the hospital? But, then, why would she call him 'Kirito'? She did not know him by that name. In fact, based on her reception of Kirito in Alfheim online, she had never heard the name before. So, why would Sugu be calling Kazuto by the name of his avatar? And why would she be using -kun if she knew she was addressing her brother? As the younger sister she would use the honorific -san or -sama, but not -kun. Understanding this he can only be hearing Asuna calling him, which is evident in the next blink when it is Asuna's face before him and not the black-haired girl. But, who was the black-haired girl and how had Kirito seen her in the first place?
Again, I can only fall back on the anomaly or glitch in the system theory and well as the strength of Kirito's own character. Only someone of extraordinary strength of character would have the capability of defying the system. Yui was attracted to both Kirito and Asuna because something about them was different from every other player in the game. Was it their love? Their strength? Their life-force? Something within their character is strong and Yui sensed it. And, as previously mentioned, Asuna also showed an anomalous ability by defying paralysis in order to protect Kirito. Was this strength enough to bypass the systems parameters so that in a near death experience Kirito could momentarily glimpse the real world and see his sister? Yes, I think it was.
But, as to who woke up Kirito - while I think he may have glimpsed Sugu, I belive he was still hearing Asuna for the reasons mentioned above, and that it was Asuna that actually woke him up.