Nurture or Nature? Is there something that really makes you "you"?

So have you ever had those moments in conversations in which you'll be talking and the other person will say, "Oh be real!" or "Stop being so fake!" They feel like you're putting up a facade or somehow not acting like your true self.

Here's the question:

Is there really some personality that is essentially you?  What makes up who you are? If you pull back all the layers, all the influences of your life (how your parents affected your personality, your friends, your education, you enviornment) is there some core something that makes you who you are?

Or... is an individual more like an onion in that when you've peeled back all the layers of his or he life, you're left with nothing, a blank slate?