5 Mind-Melting Twists to Remember From the Pretty Little Liars Finale

In the final moments, the newest serial texter lets the group know that they have Hanna. Then for the first time ever, they sign the text "A.D." But who can it be? It can't be Alison DiLaurentis, seeing as she spends most of the episode going crazy and admitting herself into psychiatric treatment. It doesn't seem like it's Mary Drake or Dr. Rollins either, they're so focused on Alison DiLaurentis and the Carrissimi group and don't mention Hanna at all. So, then, who is it?! Who is "A.D"?