[LoC: Prologue, The First Message, 59-61]
[ACoS: 12, A Morning of Victory, 348-50]
[TPoD: 16, Unexpected Absences, 332-3]
Demandred [WH: 13, Wonderful News, 314-5]
Once again, let me begin by narrowing down who she was not. Rahvin and Be'Lal were killed with balefire, so were unreachable even by the Lord of the Dark. That leaves Ishamael, Sammael, Asmodean, Aginor, and Balthamel. Ishamael was too unstable to fit. Sammael was alive still when Aran'gar first appeared. Asmodean might be a good choice, but I believe that he doesn't quite fit as well as.... you'll see. And I've already decided that Aginor is/was Osan'gar. So that leaves... Balthamel.
Okay, so that was a weak argument, but there is a lot more to this decision than a simple narrowing down. So much more is known of Aran'gar (i.e. we've seen more of her), so there is a lot more "evidence" to base this decision on. What might that evidence be, you ask?
First of all, we know that Aran'gar had once had a terrible temper: "Rage mottled in that beautiful face. She was going to do something reckless. Osan'gar knew it; there had always been a problem in that regard. Lanfear seamed cautious by contrast (LoC: Prologue, The First Message, 59-61]." Neither Aginor nor Asmodean were noted to have tempers such as this, but according to "The World of Robert Jordan's 'The Wheel of Time'": "[Balthamel] was said to have a wildfire temper that he often could not control. More than once he supposedly came very close to being bound with the Power against doing violence (TWoRJWoT, The Dark One and the Male Forsaken, 54)."
Secondly, it can be seen that Aran'gar is not happy with her placement within a female body. Could it be that the Great Lord of the Dark is displaying a bit of dark humor in his choice of Aran'gar's body? If so, then Balthamel would again be a choice victim. Consider that he was "once a good-looking man who enjoyed the company of women and was very popular with them... he spent a great deal of time in establishments that today would be called taverns of the lowest sort (TWoRJWoT, 54)." It would be a bit of poetic justice to place such a man in a body "suitable for a daien dancer in the old days (LoC: 59-61]"
Also, if the Dark One did place Balthamel into the body of a woman--a very youthful and attractive body, I might add--it might be seen as a bit of... well, generosity, on the part of the Dark One. When Balthamel emerged from the Bore, "his once handsome body had rotted to the point that he could not bear to have it seen. Unable to use his own tongue to speak, Balthamel was forced to cover every bit of his flesh and required Aginor to speak for him (TWoRJWoT, 54)."
So... who was Aran'gar? I say Balthamel.
Well, once again, I’m almost proven right. We didn’t get the big payoff in Winter’s Heart with Aran’gar that we did with Osan’gar, but we did get a few more tantalizing hints as to who she used to be. Again, there are the hints that Aran’gar had at one time been a bit too engaged in matters of the flesh, all of these coincides with my early thoughts about her being Balthamel.