Yes I don't think it's like being reborn, I think the real Sera was just a kid, but somehow she got possessed I guess you could say? All the tampering with the mirrors might have unbalanced something. Morrigan might have done something too. I feel that Sera's fighting the thoughts that belong to Andruil if we go with that theory. I have others too. She's fighting against what an elf is and stands for because of her memories and Andruil's because if she doesn't, she'll become her and Andruil's thoughts already haunt her, like her fear of Nothing and hate of Mythal. Her fear of Nothing could also be Sera's actual fear that she will become nothing if she's overtaken.
Like I said, I think we have more reasons to suspect Inquisitor himself/herself might be a vessel to *something* more than any other member of Inquisition (well, except for Solas, obviously).
I'll just... paste what I speculated about it at DA wiki forum:
I hope you forgive me this random plunge into the Sea of Speculation. I admit, this one is a rather far-fetched idea, but I can't help wondering if there's anything to it. I mean, barring some tidbits about the nature of souls and spirits I wish not to spoil to anyone who didn't get to play JOH yet, there's some info in DAI that could be interpreted as indicating that Inquisitor is more 'speshul' than we may initially think they are, aside from wielding the uber-special Anchor, that they apparently 'stole' accidentally.
It would undermine the whole "just a (relatively) normal person can establish or change the course of history" angle, which I'm not sure Bioware intends to undermine - but considering Thedas is a place where supernatural exists and where power of the will apparently can dominate everything, whether someone is more or less special in a magical sense may not be that relevant in the end.
Anyhoo, the info that made me suspect something is going on comes from - well, who else? - Solas and Cole. Everyone who befriended Solas got the scene where he wonders aloud whether the Anchor changed something within the Inquisitor; he doesn't seem to be convinced that's the case, but the apparent strength and wisdom of highly approved Inquisitor supposedly rivals something ancient. What's curious is that when Inquisitor of any background picks "I don't think I'm different from anyone" dialogue option, Solas disapproves and says that he/she isn't different only in terms of physical body.
There's even a tidbit that suggests that there's something 'spheshul' even in douchey/disapproved Inquisitors - after all, one of the first things Solas asks about in his angry scene is "Are you blissfully unaware or, deep inside, is some part of you banging on the walls, screaming?"
Cole adds to this whole confusion - he clams that the Inquisitor is "too bright", while it's been well-established that brightness and its strength is associated with powerful beings, at least in Fade-ish terms. I'd like to note that it's apparently not the Anchor's doing - it only makes the IQ even brighter.
Then there's whole Solas romance thing where he gushes about his girlfriend being unique and possessing a 'rare and marvelous spirit' - which... ok, let's assume that even Mr. Stoic has a case of looking at her through rose-colored glasses. But then we have Cole again, revealing that apparently she's real, and it means everyone could be real - that's the line that makes my brain throb.
The clues are too scarce to form a cohesive picture, but it makes me wonder whether there was a time in Thedas' (pre-Thedas?) history that existed before races - or even gods - emerged, where everyone was beautiful, bright and wholesome, only for changes to the world either decimate them or maybe prevent them from being born/reborn (if we assume that the way sentient, physical beings are 'born' are through the process Cole is going through) or made them small/incomplete/petty. That COULD explain why Solas is so baffled with the existence of Inquisitor - the one soul that apparently slipped through to the world too murky and bleak for it to either exist or thrive. Yet it did, she/he is real, therefore others could be reborn too?
Uh, I know there's very little to support it, but while it may not be what I think it is, those things are hinting towards something. And it's not just Solas and Cole - though the 'Divine' in the Fade or Flemeth/Kieran's comments about Inquisitor may be a little too cryptic/vague to extrapolate anything meaningful.
I also can't make sense out of Inquisitor glowing - no matter who drank from the Well - right before the escape from Temple of Mythal. The glow is identical to the one we see on the spectre lady in the water and appears NOT after either Morrigan or Inquisitor drinks from the Well, but only after IQ is surrounded by magical vapors emerging from the ground. Granted, I'm convinced that it is simply be a visual bug.... But is it?