And Then There was One

The Titans are gone, poof, they just vanished, but not before taking down out most of the human population, and now only a few women remain, one being deemed the 'most desirable' due to her brains and brawn, but it's not a title she cares the slightest about but Levi can't figure out how that woman is Hanji and why it's his job to protect her. How did this happen? AU-ish