Of all the crazy theories fans speculated on while watching all of those easter eggs and references in WandaVision, the one which rolled over all the web was about the existence of Mephisto in the show and the audiences' wait for him to appear as it progressed. As new characters were introduced, everyone took different digs on how one of them could be Mephisto. Starting with Dottie's character, then Agnes, and eventually the "recast" Pietro Maximoff. All of them gave villainous vibes and led fans towards establishing a prevalent comic-book villain, Mephisto, drawing parallels in the show from the comic book issues that featured the character along with Wanda and Vision.
However, out of all characters, it was Kathryn Hahn's Agnes, which people believed would turn out to be either Mephisto or a puppet or pawn of him. But, as it turned out in the penultimate episode, Agnes was revealed to be Agatha Harkness, a character who in the comics has been portrayed as one of Wanda's wicked tutors. Here, Agatha turned out to be a witch from an ancient, 17th-century coven, who is trying to grasp the limits of Wanda's powers and understand how she is managing to keep an entire town under her influence without breaking.
It's Agatha who reveals Wanda as Scarlet Witch and ends up in a conflict with her to take her powers before losing the battle. Also, all the things happening in Westview against Wanda's own wishes and magic, was Agatha All Along. From killing Sparky to "recasting" Pietro (Ralph Bohner), it was all because of Harkness, not Mephisto.
But, actress Kathryn Hahn, who played the villainous witch in WandaVision has revealed that being included in theories and comparisons to Mephisto is her favorite fan speculation about the show. Hahn, who took the Best Fight Award at MTV Awards 2021 with Elizabeth Olsen for the show, was asked about her favorite fan theories for her character during a Q&A, to which she replied as such.
"I remember hearing [from friends] that I [Agatha] was a puppet of Mephisto and I immediately had to say, 'Who is Mephisto?' Don't hold that against me! Now I know who it is."
Well, of course, we don't hold her responsible, as even writer Jac Schaeffer wasn't familiar with the comic book lore of Wanda and Vision when she was brought on board by Marvel Studios. Yet, there were too many signs of Mephisto being there, and hence, we can't blame the audience either for speculating on all those events and coming up with their own theories. The show's most obvious indication was the introduction of Billy and Tommy, Wanda's kids. The comic book lore suggests that those kids were just fragments of Mephisto, born out of Wanda's grief.
Then, there was a specific shot, where a fly-like creature can be seen in Agnes' apartment. In the comics, Mephisto is introduced first as a fly, before his demonic appearance is revealed. And then there was Evan Peters coming in as Pietro Maximoff, which led to further theories of a multiverse and X-Men in the Marvel Cinematic Universe as well.
But, despite these theories were discarded, Agatha's character turn proved worthy enough for the show to be successful and quite acceptable in the minds of fans as well.
Agatha Harkness' character sticks almost accurately to her history in the comics. Though her intentions were pretty much evil, her presence in the Hex helped Wanda escape her grief, and even got the fans a deeper look into her past. We got a pure character transformation with Elizabeth Olsen's Wanda turning into Scarlet Witch and an epic battle in the end to not just end the first season, but set up the future of both characters in the MCU, as well as the upcoming film Doctor Strange in the Multiverse of Madness.
Moreover, Kathryn Hahn did a fantastic job with the character and made Agatha an amazing villain to watch engage in conflict against her nemesis. And she is receiving all the praise for it. Yes, Hahn's favorite fan speculations didn't turn out to be fruitful, but with all those hints and easter eggs left around the Hex, let's not just rule out Mephisto from the MCU as of yet. After all, these minute details are what make this ever-expanding cinematic universe a gem to watch.