When it finally aired, it was revealed that A.D. was Alex Drake, long-lost twin of Spencer Hastings... played by Troian Bellisario.
During the time leading up to the big reveal, Troian was frequently asked who A.D. was... and guess what? She answered honestly every time.
"I never had to lie to protect the biggest secret on the show," she said to Entertainment Weekly. "I told the truth and hid in plain sight. Every time I was asked, ‘Who’s A?’ I would say, ‘It’s me,’ and everyone would laugh and think I was joking because Spencer Hastings would never be A. But I was dead serious. I just wasn’t talking about Spencer. I was talking about her twin."