The finale of Pretty Little Liars is currently airing RN, so all sorts of reveals are coming out.
The latest: Uber A's identity....
Twincer is a thing. A.D. stands for Alex Drake, the other daughter of Mary Drake.
When Mary gave birth to Spencer and the Hastings took her, Mary proceeded to give birth to a second child, who she named Alex.
The Hastings were unaware of Alex's existence and so Mary ended up putting her up for adoption.
She was then adopted by a family in England, but got "some issues" after a while, so the family gave her to an orphanage and made her change her last name back to Drake. Awks.
So, it's confirmed that it was in fact Alex who met Wren at the airport a few episodes ago.
Wren is a pretty important part of this story, and it's because of him that he knew Spencer.
Wren once actually ran into Alex in a bar in England and thought she was Spencer. They started dating, and that's how Alex discovered her relatives and Spencer, were back in Rosewood.
Alex then decided to go back to Rosewood to get ~revenge~ on Charlotte's killer (because Charlotte and Alex are half-sisters, remember).
As for that person sleeping with Toby recently. Yep it's Alex!
MORE TO COME AS THE EPISODE CONTINUES!