The brain does not have the ability to create faces, so basically its someone you have seen already.
It could be someone you saw in the street out of the corner of your eye and your brain just kept that face stored somewhere in the back of your mind.

Maybe it's the soulmate that you will later meet? Idk, I got that from a book

you must have seem her before because people in your dreams are people you've seen. it could even be some person passing by on the street.