Warning: This post contains Doctor Strange in the Multiverse of Madness spoilers
Doctor Strange in the Multiverse of Madness writer Michael Waldron addresses whether Captain Carter was the same character in the Disney+ animated television series Marvel's What if...? The Doctor Strange sequel was bolstered by hype regarding the possibilities of numerous cameos from countless heroes from Marvel Comics' library. Recent promotional material teased the inclusion of the super serum-enhanced variant of Peggy Carter (Hayley Atwell). Fans' suspicions were confirmed when Captain Carter took a seat on the council of the Illiuminati, a group of superheroes in the alternate 838 universe.
The character of Captain Carter first appeared within the MCU animated anthology series What If...? that explored alternate timelines within the multiverse if different actions were taken in key moments of the franchise. In the first episode of the series, Agent Carter was injected with the super serum following a grave injury to Steve Rogers. Following her genetic enhancement, she takes on Hydra, but during an attempt to protect Rogers, she gets sucked into an inter-dimensional portal and ends up traveling 70 years into the future where she runs into Nick Fury and Clint Barton. A live-action Captain Carter was featured in some Doctor Strange in the Multiverse of Madness marketing materials, causing some to wonder if it's the same character as What If...?.
In a recent interview with IGN, Doctor Strange in the Multiverse of Madness writer Michael Waldron discussed Captain Carter's appearance in the film. When asked if she was the same character as the one from the What If...? series, Waldron responded with ambivalence. Read what the writer said below.
"I don't know. I'm reticent to canonize that either way. I hope not. I hope that's not the one. That'd be too bad."
The Doctor Strange sequel provides no evidence to the idea that both the live-action and animated versions of Captain Carter are the same characters. Although they are played by the same actor and their costumes and shields appear to be similar in design, the film offered no references to the animated universe and story. Waldron's desire that the characters are different is most likely a response to the less-than-fortunate fate of the feature film version.
Although it is unlikely that Earth-838's version of Captain Carter is the same as the one in What If...?, it is possible. Due to Agent Carter's decision to take the super serum herself instead of Rogers, all of Earth's history following that event has been forever changed, possibly setting up the Earth-838's world. When Captain Carter arrives in the future, it is unclear how the surrounding world has deviated from Earth-616's original timeline. It is possible that the Fantastic Four and the X-Men have formed due to the repercussions of Carter and Zola's actions in the past. Regardless, barring another season of What If...?, Captain Carter's reappearance in a future MCU film, or additional specificity from Marvel Studios president Kevin Feige himself, Doctor Strange in the Multiverse of Madness has no definitive answer to that question.
Source: IGN
