There's only one thing more mysterious than A on Pretty Little Liars: the ABC Family previews for each upcoming episode.

Next week, for example, we know that an unexpected ally surfaces to help the girls out on "Know Your Frenemies." But the following video doesn't give us any idea who that will be.

Form your own theories as you read through our latest Pretty Little Liars review and see what you can surmise based on the official trailer here:

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.