Sadie Sink is officially trading the Upside Down for the Marvel multiverse. After breaking out as Max in Stranger Things, the actor is set to make her Marvel Cinematic Universe debut in Spider Man: Brand New Day and, according to new reports, will carry that mystery role straight into Avengers: Secret Wars, the event film that is expected to reshape the MCU.
From Hawkins To The Avengers
Sadie Sink was first linked to Spider Man: Brand New Day earlier this year, when she was cast in a significant but secret role opposite Tom Holland and Zendaya. The film, directed by Destin Daniel Cretton, is framed as a fresh start for Peter Parker after the memory wipe at the end of No Way Home, with Peter rebuilding his life and staying closer to street level threats in New York. Brand New Day is scheduled to hit theaters on July 31, 2026, as part of Phase Six.
What we still do not know is who Sink is playing. Fan speculation has gone in every direction, from classic Spider Man supporting characters like Gwen Stacy and Mary Jane Watson to more left field picks such as Shathra or even an original character created just for the MCU. The only consistent thread is that trade reports keep describing her part as major, which suggests she is not just another classmate or quick cameo.
The Secret Wars Connection
The new wrinkle is that Sink is expected to stay in the MCU long after her Spider Man debut. A recent report drawing from a Deadline column notes that she will join the cast of the next Avengers film that shoots in London in 2026, which lines up with Avengers: Secret Wars, the sequel to Avengers: Doomsday and the planned capstone to the Multiverse Saga. Other coverage has already started framing this as Sadie Sink being part of Secret Wars, with filming eyed for the latter half of 2026 and release set for December 17, 2027.
Doomsday, which arrives in December 2026, is bringing together a wild roster that includes returning Avengers, the Fantastic Four, the New Avengers lineup from Thunderbolts, and legacy X Men actors from the Fox films, all facing Robert Downey Jr in a new role as Doctor Doom. Secret Wars is being shot after Doomsday and is designed as the true finale for this era of Marvel, with multiverse crossovers and a likely soft reboot of the universe on the other side.
If Sadie Sink is already booked for Secret Wars before fans even know who she plays in Brand New Day, that is a clear signal that Marvel sees her character as part of the long term picture, not a one off guest star.
Why Fans Keep Pointing To Jean Grey
The biggest fan theory right now is that Sink is playing Jean Grey. That idea has been floating around since she was first attached to Brand New Day, largely because Marvel is finally ramping up its own X Men reboot and has hinted at a younger, more long term cast of mutants. Trade reports have confirmed that Marvel’s X Men film will be written by Michael Lesslie and directed by Thunderbolts director Jake Schreier, with a focus on a new generation and a more youthful tone.
Sadie Sink fits that strategy almost perfectly. She is in her early twenties, already has genre cred from Stranger Things and Fear Street, and has shown she can handle big emotional swings, which is essential if Marvel wants to eventually do justice to storylines like the Dark Phoenix saga. If Marvel introduces Jean Grey in Spider Man: Brand New Day as a supporting player, then moves her into Secret Wars, it would neatly bridge the current Avengers era into the mutant future that is waiting on the other side of that multiverse reset.
Of course, that is still speculation. Reports so far only confirm that she has a major role in Brand New Day and that she will return in the next Avengers film, not which character she is playing. Some coverage has already pushed back on specific theories like Shathra, while others keep Jean Grey, Gwen Stacy, and even Mayday Parker in the conversation. Until a trailer drops or Marvel makes things official, her exact identity stays firmly in rumor territory.
Until Marvel finally reveals who she is playing, the safest bet is this: if Sadie Sink is important enough to swing through Spider Man and stand in the middle of Secret Wars, she is going to be one of the characters to watch as the MCU resets itself and starts building its next decade of stories.
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