Warning! Spoilers ahead for the WandaVision finale.
WandaVision episode 9, "The Series Finale" didn't reveal Mephisto as the show's secret villain - and it's for the best. Marvel Studios' inaugural MCU Phase 4 series has finally come to an end with a more traditional climactic battle reminiscent of its usual fare. That's after it started as a sitcom-inspired TV show that felt and looked nothing like what's been in the franchise thus far.
Over the course of its run, the series' true narrative became increasingly clear. The penultimate outing traced back some of the most traumatic experiences of Wanda's life leading to the revelation of how she was exactly able to create the hex. Following its reveal that Agnes aka. Agatha Harkness was the primary antagonist, there were still some who were convinced that the show was planning a final plot twist revealing her to be in cahoots with another bad guy. Amidst the string of possibilities, Mephisto was the frontrunner with his speculated involvement backed up by subtle references to the devil peppered throughout the show. But, ultimately, the show stuck to its episode 7, "Breaking the Fourth Wall" reveal that Agatha has been WandaVision's real villain all along.
So why didn't Marvel Studios pull another plot twist in the WandaVision finale by having Mephisto finally show up? After all, there seems to be a lot of excitement over the prospect of seeing his debut in the MCU. It's because Marvel Studios prioritized its story rather than another surprise character reveal. At its core, WandaVision's narrative was about Wanda's grief — something that's integral in the creation of the Hex which lured Agatha to Westview in the first place. Despite the string of demon clues in the show, there was no concrete setup for Mephisto to show up. Unlike in the comics, Wanda's kids Billy and Tommy weren't magical constructs made from shards of Mephisto's soul. Instead, they were creations of Wanda's new reality, meaning that not only were they not real, but they have no connection to the devil. Debuting him at the very last episode of WandaVision and revealing that Mephisto was its true villain would feel forced and unearned. In any case, Agatha was powerful and compelling enough to be the show's primary antagonist.
The series referencing the devil countless times was a deliberate move to keep viewers distracted by the possibility while going in a totally different direction. This checks out with Marvel Studios' general storytelling principle. WandaVision was full of red herrings, which includes the stunt casting of Evan Peters' as fake Pietro, leading many to believe that it's the first step towards integrating the Fox X-Men franchise into the MCU. It helped that the show was expected to be a cornerstone of its plans to establish the multiverse which would've made this scenario entirely plausible. In the end, nothing came out of it, and with that trick already used, repeating the same scheme with the surprise reveal for Mephisto in the WandaVision finale would've felt tedious, if not irritating.
In hindsight, this was for the best as it kept WandaVision's narrative character-driven; it was solely focused on Wanda's transformation as Scarlet Witch and her subsequent farewell to her family as the Hex came down. Granted that Mephisto's arrival could've been a set-up for upcoming MCU movies and shows such as Loki, it would've taken away the attention from its core story, the same way Luke Skywalker's The Mandalorian season 2 finale appearance somewhat distracted from the implications of Din Djarin and Baby Yoda's separation. The MCU has been burned before by overdoing setting up their future as seen in films like Iron Man 2 and Avengers: Age of Ultron; it seems like they've finally learned their lesson this time.
