I can tell you. Sophie's real name is Sophie.
See, I don't care what it says on her birth certificate. I don't care what it says on the most legal of her legal documents. The four people she works with most, the four people she appears to care about most, call her Sophie. And therefore her name is Sophie. She may also have another real name. But Sophie is her real name.
I have a friend named Ctein, and from conversations with him I understand he has gone through the legal process to make sure that all documents reflect this. That is his name. But guess what? If the US government thought his name was John Smith or Susan Goldstein or Bobatundae Chen, his real name would still be Ctein. Because when I see him, I say, "Hey, Ctein!", and he says, "Hi!" and gives me a hug. If instead he sighed and said, "I told you not to call me that. Please call me Larry," and he wasn't joking, then his real name would be Larry. But it's not, it's Ctein.
One of my friends posted a list on Facebook about being polite to transgendered people, and as far as I could tell, the rule of thumb was, "Assume transgendered people are people; proceed." Because I could not think of any other circumstance in which someone would say, "Hello, my name is Jennifer," and it would be polite to respond with, "But what is it REALLY? What is your REAL name?" If someone says, "Hello, my name is Jennifer," the polite response is, "Pleased to meet you, Jennifer." Or possibly, "Duck, Jennifer, there are ninjas coming at your head!" But mostly the former. The latter circumstance rarely arises but is still more commonly polite than quizzing Jennifer on her REAL name.
The stories where finding out who somebody used to be turns out to be the most important thing--I can connect to those sometimes. I can connect to those when who that person used to be was forcibly taken from them. So, like, slave narratives. Or some Native American/First Nations stories. Times when someone had to hide. Times when they've forgotten who they used to be, although those are iffy for me because they're so often done badly. But when someone has had a good degree of free choice, I feel pretty strongly about respecting that free choice. If they've left a parent of origin who was pretty crappy, sometimes they have kept the name that parent gave them, and sometimes they've picked a new one, and I don't feel that insisting that the real true them is not the interesting person they've built, but rather the childhood they left, is a good idea. I feel that that's disrespectful of the person they've built and the choices they've made. And even when it's not a matter of crappy parenting--yes, the past is important. It contains our roots. But I think we can get way too caught up in that. In the case of "what is Sophie's real name?", it's a very superficial rooting. We have no ongoing character Sophie could turn out to secretly be, no history that could turn out to suddenly be hers. We already met her family of origin. We could just now discover that they originally named her Madeleine or Mehitabel or Claudia or Claire. Which...would not make her not-Sophie in any way that I can see.
So I tend to bounce off of secret name magic in fantasy, and I'm bouncing off it even more in Leverage, where it has no built-up use. The kennings and cognomens that we have--they are knowable. They are reachable. The way we have them is by being ourselves. You cannot keep them secret, because you walk through the world making a hole that is shaped like Sophie, or like Ctein, or like Mrissahainen mighty-sinewed chemist's daughter, or like you. And the parts of yourself that are secret to you can only stay really completely secret for so long before they reach out and shape bits of glass into tiny worlds--and even when they do, the people who have been around you will nod and say, well, that's the sort of thing, you know. I didn't know it would be that world in specific. But I suspected she might. There will be someone who is not in the least surprised, because you have been going around being you, and even when it's an unpredictable kind of you, there will at the very least be people who are smart enough not to try to predict, after awhile.
Maybe that's just in my life. Maybe this Sophie's-name thing is working for people because they have lives full of people who persist in trying to predict in detail and being surprised when it comes out funny. Or maybe there are some of you for whom the secret name thing works better, and you'll be able to tell me why in comments.