One of the joys of WandaVision is watching the rollout of all the characters in Westview. For example, in the first episode, viewers met Phil Jones, Vision's sad-sack colleague. In Episode 2, the show introduced Dottie, the neighborhood queen bee. Then, Episode 3 revealed the two are married, which is just a hilarious punchline. But even as the neighbors all start connecting, there's one person fans haven't yet seen: Agnes' husband, Ralph. It's led to many theories, including one that says WandaVision's Ralph is actually Marvel villain Mephisto.
Warning: Spoilers through WandaVision Season 1, Episode 3 follow. Fans know there's no way this Nick-at-Nite version of Wanda's life is real. But other than a few extraneous commercial breaks and a hint or two about the sitcom not being all that it's cracked up to be, WandaVision plays this reality surprisingly straight. That's due to Wanda; when this made-up world starts to misbehave (like when random beekeepers come out of manholes), she rewinds and resets the show.
Wanda's control over the world has led to the theory that this version of reality is a pocket universe Wanda created to escape her grief over Vision's death. That theory got another boost when Monica Rambeau broke her cover as Geraldine in a bid to connect with Wanda. Her decision backfired; Wanda rejected the overture, booting her back to the S.W.O.R.D. base from whence she came.
But Wanda may not be as in control of the situation as it appears. Up until Episode 3, Vision was utterly under her control. If he saw something that was off, Wanda wiped the inconsistency from the narrative straight away. But Wanda was too focused on Geraldine to notice Vision started questioning the nature of his reality (thanks to Agnes).
Agnes has been an enigma since the series started. She's always watching Wanda, popping up with pineapples and rabbits. She chatters on incessantly about her awful husband Ralph, who, so far, neither Vision nor Wanda have seen. But in the endless chit-chat, fans have glommed onto one odd statement. When someone mentioned the devil's in the details, Agnes responded: "That's not the only place he is."
For this reason (and a few others), fans suspect Agnes may be Agatha Harkness, the Scarlet Witch's mentor. But Harkness has a second connection to Wanda, too. As a bonafide witch, she's connected to Mephisto, an evil creature from a different multiverse.
As the lore goes, Mephisto takes on the devil's form to make Earthlings fear and worship him; sometimes, he even bestows them powers. (In 2007's Ghost Rider's Johnny Blaze "sold his soul to the devil." He was dealing with Mephisto.) In the comics, Mephisto does terrible things to Wanda, including killing her children — who just so happen to be named Billy and Tommy (like her new babies in WandaVision).
If Mephisto is, in fact, in WandaVision, it could explain the show's connection to Doctor Strange, as Mephisto is a primary antagonist in the sorcerer's comics. WandaVision was initially planned as a lead-in to the Multiverse of Madness, and Elizabeth Olsen, who plays Wanda, is confirmed to be appearing in the film. Having WandaVision be the introduction to the film's main villain would explain a lot.
Fans will have to wait until Episode 4 to see if Mephisto is, in fact, Agnes' husband Ralph. Best to keep an eye out and see if the devil really is in WandaVision's details.