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Aubrey Plaza makes her MCU debut in “Agatha All Along,” but so far her character Rio Vidal has been shrouded in mystery.

This, of course, encourages fan theories – and Plaza herself has fanned the flames of speculation by addressing the mystery surrounding her character. “I’m just so happy that they’re letting me play this character,” she told Scarlet Witch Updates. “I can’t give anything away, but what happens, the reveal is pretty amazing. And I think, especially, the fans are going to go crazy.”

WandaVision sparked a ton of theories when it was released on Disney Plus, so it’s no surprise that Plaza’s comments sparked the resurfacing of an old joke. “I call it: Mephisto,” says one person: There was heavy speculation that the villain would appear in WandaVision, but despite all the fan theories, he never showed up.

“Aubrey Plaza as… Mephistopheles,” jokes another fan. Another person Echoes: “Holy shit, guys, Aubrey Plaza is going to play Mephisto”

Of course, other, more serious theories have also emerged. “Rio Vidal means river of life and refers to the first green witch Gaia, also known as Mother Earth. Aubrey Plaza will most likely play her in the MCU and not Mephisto.” speculates one person.

“Aubrey Plaza plays Cynthia Von Doom in Agatha All Along. Bookmark this tweet.” suspects another fan. “It’s just a guess, nothing is confirmed, but I think it makes a lot of sense considering Avengers Doomsday is coming soon and how important his mother is to his story. She was a witch and has ties to Mephisto, who is rumored to appear in the series as well.”

“Morgan le Fay, obviously. She even looks like her.” says someone else. We won’t have long to find out if any of these theories are true, either, as the first two episodes of Agatha All Along premiere on Disney Plus on September 18.

While you wait, you can check out our guide to all the upcoming Marvel movies and series to find out everything else the MCU has in store, or plan a marathon with our explainer on how to watch the Marvel movies in order. But if you’re short on time, try the Marvel movies and series to watch before Agatha All Along.