we are trying to identify people, we normally use it to refer
to them, so in your example, Fabiola, the answer would be: It's
Susan, not She's Susan.
Incidentally, for the question, we would probably use the definite
article and say: Who's the girl in the blue T-shirt? as we
are talking about a particular girl and a particular T-shirt.
are some more examples of it used to identify people:
that Prince Harry talking to the TV reporter? ~ No,
it isn't. It's Prince William.
must go. Let's get the bill. Is that our waitress over there?
~ No, it isn't. Our waitress has got longer hair.
in a face-to-face situation which is more personal, we sometimes
use the second person pronoun instead of the third person.
Compare the following:
We've met before, haven't we? It's Tom, isn't it? ~ Yes,
We've met before, haven't we? You’re Tom, aren't you? ~
Yes, I am.
My name's Susan and you are...? ~ I'm Rachel.