The Walking Dead fully unmasked Whisperer Gamma (Thora Birch), real name Mary, when she opened up about her hazy past to Judith Grimes (Cailey Fleming) in Sunday's "Stalker." The once loyal acolyte of Alpha's (Samantha Morton) turned on the Whisperer leader upon discovering she lied about killing daughter Lydia (Cassady McClincy) — who showed up at the Whisperer border in "Open Your Eyes," by way of cunning maneuvering from Carol (Melissa McBride) — a revelation that convinced Mary to reach a deal with Aaron (Ross Marquand), supplying the Alexandrians with the location of Alpha's walker horde in exchange for a chance to reunite with her last-surviving relative.
Mary made a second dangerous deal when she crossed enemy lines to reach Alexandria, where she was taken into custody by a suspicious Rosita (Christian Serratos) and Father Gabriel (Seth Gilliam). For a chance to see her nephew — baby Adam, adopted by Tammy (Brett Butler) and Earl Sutton (John Finn) after being abandoned outside Hilltop — Mary would identify the cave holding Aaron and other heroes trapped by Alpha's horde, not knowing most of them managed to escape.
Locked in the cell that once held Negan (Jeffrey Dean Morgan) and Whisperer spy Dante (Juan Javier Cardenas), whose blood still stains the cell floor after being viciously stabbed to death by Gabriel, a lonesome Mary was visited by a curious Judith perched outside. After introducing herself as Mary, and noting that was the first time anyone asked her name since arriving, Mary divulged she's from Santa Monica — "That was in California" — born to parents John and Alice.
"I went to school, and... I don't remember," Mary admitted. "Alpha has a way of making you forget." There are just "echoes left," she added, "feelings." She was ordinary, and so was Frances (Juliet Brett), the sister Mary killed when protecting Alpha in "We Are the End of the World."
"When it all went down, my sister and I kept making bad decisions," Mary said. "We needed someone to make them for us, and then we met Alpha."
Judith, in her earnest innocence, said Mary "met the wrong person first."
"If you met my mom and my dad," Judith said of Michonne (Danai Gurira) and Rick Grimes (Andrew Lincoln), "you wouldn't be in that cell. You don't look like a monster to me."
When explaining Mary's "split-second" decision to save Alpha at the cost of her own sister's life earlier this season, Birch said it was a decision made in line with the indoctrinated Whisperer's belief that the pact must protect their alpha.
"I think in that moment it's very much an instantaneous decision, she doesn't even think about it. It's very much, 'My plane is going down, who do I protect? Do I protect the co-pilot or the pilot? I protect the pilot,'" Birch said on a past episode of Talking Dead. "And I think that that's very much her thought process. She doesn't even think about it. It's not even her sister on top of Alpha, ready to kill her, it's just an entity. And we have to remove that entity to protect the god."
Mary has since come to fully regret that decision: she tearfully confessed to her sister's murder when interrogated by Father Gabriel, telling him, "I killed my sister. I killed my sister. I killed her. I killed her for Alpha. She didn't even have to ask… I just did it."