WARNING: The following contains spoilers for WandaVision Episode 6, "All-New Halloween Spooktacular!," now streaming on Disney+.
Last week, Disney+'s WandaVision brought Evan Peters' Quicksilver from the X-Men franchise into the Marvel Cinematic Universe as the new Pietro Maximoff. Although some believe this to be a step towards introducing the multiverse, this character may not actually be who he claims to be. Instead, he could be a villain disguised as Pietro, and he infiltrated Westview to gain Wanda's trust.
As Wanda points out, there are several differences between this Pietro and the one she grew up with. While he does have the same power of super speed, the new Pietro has an entirely different face and is also missing his Sokovian accent. Wanda herself lost her accent over time, but it does occasionally come back, usually when she remembers something important from her past. Conversely, Pietro's accent hasn't returned at all yet.
Furthermore, his personality and mannerisms don't match those of Aaron Taylor-Johnson's Pietro. They're more of a mixture between Peters' Quicksilver and those of an uncle character in a sitcom. Even underneath this persona, there's still a lot to uncover about his character.
This Pietro also demonstrates a level of awareness not shown by anyone else in Westview. He claims not to know how he came to be here or why he looks different, but he knows that he died and that Wanda is controlling the town. This stands in stark contrast to Vision, who remembers nothing from before Westview and has had to slowly put the pieces together. For instance, in this episode, Vision found out from Agnes that he died, but Pietro already knew this, despite the fact that he died before Vision.
Wanda also tries to test him after realizing his memories from growing up don't match hers. One of the only things Pietro doesn't know is how Wanda created the Hex in the first place, a topic he shows much interest in. He may just have been curious about his sister's powers, but he could also have an ulterior motive. If he is a villain, he could've been gathering information about Wanda's powers for future nefarious plans.
It should also be noted that when Pietro arrived in the show, S.W.O.R.D.'s alarms go off, indicating someone crossed the barrier between Westview and the outside world. So far, the only people who have left Westview are Monica, who was thrown out; Wanda, who left to confront S.W.O.R.D. and Vision, who started to disintegrate as soon as he left. Since they didn't find anyone else trying to leave the Hex, the alarms must've gone off because Pietro entered from the outside.
Unlike Monica, who fell under Wanda's mind control upon entering, Pietro seems to have kept control over his mind. In other words, he might be powerful enough to resist Wanda's abilities. This would explain why he can speak openly about his past and his role in the show, something only a few others have managed to do.
If Pietro is a villain in disguise, there are a few possible identities. One theory is that he's Nightmare, a Doctor Strange villain. In the comics, Nightmare is a demon who can control people when they sleep by capturing their astral forms and bringing them to his realm to torture them. Perhaps the Westview anomaly is an extension of Nightmare's realm, and the townspeople are all asleep. After all, Pietro postulates the children of Westview haven't appeared until now because they may be asleep, and he describes Wanda's powers back in Age of Ultron as nightmare inducing.
Another theory is that he could be Loki. More specifically, he might be the Loki who escaped with the Tesseract in Endgame and will be captured by the Time Variance Authority in his upcoming series. Although the series is a few months away, it seems that Loki will be causing mischief across different realities. He may want to recruit Wanda to help him take down the TVA, offering her a Mind Stone from a different reality to rebuild Vision. He may also have chosen to disguise himself as Pietro, not realizing that the Pietro of this reality looks different from the one he met in an alternate universe.
Finally, he could be the villain whose appearance fans have been predicting since the beginning: Mephisto. Marvel's devil has tried to steal Wanda's magic in the comics, and her sons were made from pieces of his soul. His powers include shapeshifting and super speed, which would easily allow him to pose as Quicksilver. In Episode 6, Pietro calls the twins "demon spawn" and says that Westview is "charming as hell." While his wording could be just be part of his sitcom persona, it may also be a subtle hint at his true identity.
Written by Jac Schaeffer and directed by Matt Shakman, WandaVision stars Elizabeth Olsen as Wanda Maximoff/Scarlet Witch, Paul Bettany as Vision, Randall Park as Agent Jimmy Woo, Kat Dennings as Darcy Lewis, Teyonah Parris as Monica Rambeau and Kathryn Hahn as Agnes. New episodes air Fridays on Disney+.