The Best Explanation: Wren has always been deeply woven into Radley Sanitarium. He worked at the hospital while both Charlotte and Mona were patients there. There's a strong possibility he was in love with Charlotte, or at the very least took a special liking to her. And, honestly, who knows where else he might be involved in this mystery? His British accent ties him to Archer Dunhill (who, by the way, has those ominous A.D. initials). His age and tendency to hit on underage girls could mean he somehow met Ali. Any of these tenuous tie-ins could provide fuel for Wren's mission.
The Most Damning Evidence: Wren has always been presented as a shady character. He hits on Spencer while he's engaged to Melissa, and he even flirts with Hanna at one point. He's a doctor, which would explain a lot of the medical abilities "A.D." seems to possess. We even see Wren in season seven, having a rather intense conversation with a pretty manic Spencer (or . . . Spencer's twin, right?). The fact that he's still talking to Spencer, and Melissa for that matter, means he can be linked to two other suspects on this list. Even if "A.D." doesn't end up being Wren, I find it almost impossible that he's not somehow involved.
