A lot of people seem to think that "The Girl" is most likely Serena. There are a few problems that I see with this, but maybe someone could explain a way to resolve them.
Unless we have some kind of misconception about how people get into the city, Serena being there would imply that her aura was absorbed by Rhue's sword, Phantom Slasher. However, this would mean that the sword that Jeruh killed Serena with is Phantom Slasher and that Rhue obtained it sometime after Jeruh lost it. The problem I see with this is that there are things that indicate that "Rhue" has been around much longer than this. Examples of this are Kavax (Kava) recognizing Rhue as Kura and Gaius telling Rhue that he thinks he has been around for a long time. If The Girl really is Serena then the only explanation I can think of is that Rhue (whoever he was at this time) actually had the sword before Jeruh obtained it and was only temporally separated from it like we have seen several times throughout the series.
The other problem is that Serena was a little girl when she was killed. A lot of people think that the inhabitants of the city can age, but there are also problems I see with this. There are many things that indicate that time passes very slowly in the city. Lyrra should have been there for the shortest amount of time, but she remembers being there for years. Jed says he was there for as long as he can remember. In Episode 6, every thing that Rhue does in the city only seems to take only a few moments of "real time". However all the characters that we had seen outside of the city appear to be the same age as they were when they died, characters that we know have been there for a very long time (Kavax) do not appear to be old, and there are even children in the city. I don't see why Serena would be the only one to have aged.
I would also like to ask about the evidence for The Girl being Serena. The only thing I remember is that they both have blonde hair. If there is more evidence that I am forgetting then please post it.