The character of Mephisto ties directly into some of the biggest theories we have so far about WandaVision, but he's never actually been mentioned on screen at all! While we're trying to figure out exactly what's happening in Westview and who's in control of everything, it's the perfect time to consider Mephisto's comics history and the very real possibility that he's about to be introduced as the newest baddie of the Marvel universe.
In the comics, Mephisto is, pretty much, the Marvel version of the devil. He's one of the biggest "Big Bads" there is, and pretty much all of the heroes come into conflict with him at some point. He's made of the same nebulous cosmic energy as the Infinity Stones, and, at one point, tries to guide Thanos into collecting the Stones and causing mass destruction. In the comics, he's able to personally acquire the Time Stone, which — wait for it — allows him to travel through time and the multiverse, creating alternate versions of himself so that, even if he's killed or destroyed, an alternate self can fill the void. That sounds like the kind of story that would fit the cryptic teases for Doctor Strange in the Multiverse of Madness, which will feature Wanda in a pivotal role.
Crucially for WandaVision, Mephisto plays a major role in a tragic storyline that it looks like the show is borrowing from: the birth of Wanda and Vision's sons. In the comics, as in the show, Wanda and Vision somehow have twin sons despite Vision being an android — but, tragically, it's revealed that the boys are actually formed from fragments of Mephisto's soul, so when he reabsorbs those fragments, Wanda's sons disappear and her sanity slowly deteriorates.
There are more than a few hints that Mephisto may be making an appearance in the Marvel films and TV shows sooner rather than later. In the trailer for Loki, a split-second shot seems to show a stained-glass window depicting a figure that looks an awful lot like Mephisto's appearance in the comics. In WandaVision's second episode, there's a seemingly throwaway exchange that, in hindsight, seems a lot like foreshadowing. One neighbour quips, "The devil's in the details" to Wanda and her mysterious neighbour Agnes, and Agnes, in turn, jokes, "That's not the only place he is!" Agnes often jokes about her husband Ralph, who's constantly the butt of the joke but never has been seen on screen. Initially, it seemed like a running gag in the style of old-school sitcoms, but it could also have more sinister motivations.
Considering Mephisto's ties to Wanda's story and to the multiverse (which we already suspect will play a big part in the next phase of Marvel stories), it wouldn't be surprising at all if he's revealed as the mastermind and future long-term villain.