WARNING: The following contains spoilers for WandaVision Episode 8, "Previously On," now streaming on Disney+.
The latest episode of Disney+'s WandaVision confirmed something several fans have suspected from the start: Evan Peters is not Marvel's new Pietro Maximoff. Instead, he's posing as Wanda's brother and was sent by Agnes -- Agatha Harkness -- to find out how Wanda created the Westview anomaly in the first place, but this leaves viewers with a couple of important questions, like who is this character and why is he working with Agatha.
Episode 8 reveals Wanda is wielding chaos magic, something that enables her to seemingly manifest things out of thin air. This is how she's able to create the Hex, as well as a new Vision without the Mind Stone or his original body. Agatha claims it takes regular witches years of training to learn to cast simple spells, so the ability to create something out of nothing is unique to Wanda. In other words, Agatha could not have created Fake Pietro from nothing like Wanda did with Vision, so he's a separate entity whose true identity is still a secret.
Although he appears to be Agatha's pawn, Fake Pietro's true identity could still be important, and there are a few possible identities that come to mind. As many have theorized, he could be Marvel's devil, Mephisto. After all, Wanda's twin sons were made from pieces of his broken soul in the comics, so introducing him along with Billy and Tommy would make sense. There are also quite a few Satanic images incorporated into the design of Agatha's basement.
However, with only one episode left in the season, it's getting rather late to introduce such a significant villain. Plus, this might overshadow Agatha's role as the main antagonist, and it wouldn't explain why Agatha was using her own magic on him as shown in her "Agatha All Along" sequence. If Mephisto does appear in WandaVision, it seems more likely that he'd be revealed as the bigger villain behind the scenes, like Thanos' first appearance in The Avengers.
Another possible identity is Agatha's son, Nicholas Scratch. His origins are rather mysterious, but he was once the leader of New Salem, a witch community in the comics. Later, Scratch turned the community against his mother before he himself was banished. This could be an interesting way to introduce Scratch into the Marvel Cinematic Universe by having him work with his mother before ultimately betraying her.
Some have even speculated that Agatha turns Peters' character into her pet rabbit, Señor Scratchy, when he's not pretending to be Pietro. At the same time, this would not explain why S.W.O.R.D.'s alarms went off, signifying that the barrier had been breached, when he first appeared. This event implies that Peters' character entered Westview right at that moment, whereas Señor Scratchy has been inside the Hex since at least Episode 2. Even so, this doesn't necessarily rule out him being Nicholas Scratch.
On the other hand, Agatha mentions that Fake Pietro's her "eyes and ears" and calls her spell on him "a crystalline possession." This appears to be a new term not directly based on anything from the source material, but it suggests that Agatha is controlling him to a certain extent. If she is fully possessing him, he could just be another random Westview resident who Agatha picked out to do her bidding. This would explain why he was aware of his sitcom role and why he knew things the real Pietro wouldn't have known, like the fact that Vision died.
On the other hand, this still doesn't account for S.W.O.R.D.'s alarms going off. It seems that all Westview residents either became part of the Hex or forgot the town existed depending on where they were when the Hex went into effect. Additionally, Agatha would've had to find a way to replicate Quicksilver's superspeed, and whether or not that's within her power is currently unclear.
Finally, it could turn out to be Peter Maximoff from Fox's X-Men universe, proving that fans' first instincts about the character were right all along. If Agatha's able to possess him, she would've given him all the information he needed on Wanda, Vision and the Hex. He also would still have his superspeed and would've tripped S.W.O.R.D.'s alarms when he entered the Hex.
The main unknowns in this case would be how he got into the MCU and why he came to Westview. If Episode 7's commercial is referring to the Nexus of All Realities, Peter could've entered the universe through the Nexus and just happened upon the Hex. Alternatively, Agatha could've kidnapped him from his home universe if she had access to the multiverse. Either way, this could formally introduce Marvel's multiverse, providing a connection to the upcoming Doctor Strange sequel.
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+.