Fans Think Rosalina’s Super Mario Galaxy Movie Role Will Confirm A Long-Standing Theory About Brie Larson’s Character

The recently announced Super Mario Galaxy Movie hasn’t even landed yet, but fans are already connecting dots faster than Mario on a Rainbow Road speed run. The first trailer for Nintendo and Illumination’s cosmic sequel gave viewers everything they hoped for—floating planetoids, Lumas, Launch Stars, and the long-awaited debut of Rosalina. But the real lightning strike came with the casting reveal that Brie Larson is voicing the mysterious cosmic princess. And for fans, that instantly reignited one of the Mario franchise’s biggest, oldest theories.

According to reporting from ComicBook.com, Rosalina’s expanded presence in the trailer—as well as Larson’s A-list casting—strongly suggests the film may finally address a question fans have debated since 2007: Is Princess Peach secretly Rosalina’s sister? It’s a theory supported by years of fan analysis, developer comments, and a surprisingly lore-rich in-world book. Now, the movie may be ready to make it canon.

In the Super Mario Galaxy video game, Rosalina’s backstory is revealed through a storybook that describes her leaving her home world after her mother passes—depicted as “sleeping under the tree on the hill.” The book also makes it clear that she grew up with a father and a brother. Fans have long zeroed in on a dreamy illustration of her mother, noting the character’s Peach-like features, from her hairstyle to her jewelry. Adding more fuel to the theory, the game’s official Prima guide once noted that the developers initially planned for Rosalina and Peach to be related. However, that idea was dropped before the game shipped.

The Super Mario Bros. Movie only supercharged the theory. In the 2023 film, Peach is explicitly rewritten as a human child who entered the Mushroom Kingdom through a warp pipe—an origin she shares with Rosalina’s established backstory. Was that just a cute twist? Many fans don't believe so, but instead think it was the groundwork for a bigger reveal in the sequel.

Thematically, the two characters already align. Both were separated from their families as children, went on to rule worlds far from home, and embody the “lost star-child” archetype that defines the Galaxy era of Mario. Fans argue that formally connecting them would deepen the mythology and give Peach a personal arc that moves beyond the damsel, ruler, and mentor roles into something far more emotional.

And then there’s Brie Larson. Casting someone of her caliber for Rosalina doesn’t just elevate the character's status but also signals her narrative weight. You don’t bring in an Oscar-winning actress to quietly orbit the edges of Mario’s journey. Her presence suggests a story that’s both cosmic and personal—and potentially answers questions fans have been holding onto for nearly two decades.

Of course, all of this remains theory until Nintendo makes it official. Still, the trailer’s tone, Peach’s revised movie origin, and Rosalina’s expanded role seem to be pointing in the same direction: the long-running sister theory may finally be leveling up from fan speculation to on-screen canon.

We’ll get answers soon enough when The Super Mario Galaxy Movie joins the 2026 movie release schedule on April 3, 2026. Until then, the speculation is only getting louder—and Brie Larson’s take on Rosalina may be the key to unlocking the Mario universe’s most emotional family reveal yet.

While we wait to learn all of Princess Rosalina's secrets, fans can revisit the series’ first installment. The Super Mario Bros. Movie is available to stream with an Amazon Prime subscription.