Where she came from is clearly explained in the book.
I know this is a story that has been done before in sci-fi: suddenly there's a new character and all the old characters are acting as if the new character was there the whole time even though the audience knows that's not right. See Buffy the Vampire Slayer's little sister Dawn. Or the Farscape episode "Rapsody in Blue". Still, I like that story trope. It's interesting. And we get to "meet" a new character.