Fans of the Marvel Cinematic Universe were glad to see Natalie Portman's Jane Foster back in action in the first trailer for Thor: Love and Thunder. She definitely needed closure on her arc and redemption after being treated like a constant damsel in distress in the first two movies and cut from Ragnarok totally. And to make things sweeter, she was kicking butt in her Thor form, rocking a rebuilt Mjolnir and wowing her ex-boyfriend. However, as fun as it is thinking about how she'll be an ally when the God of Thunder goes up against Gorr, there are hints that this version of Jane might not be a hero.
It would make sense for Jane to return to help a lonely Thor find inner peace and his overall identity. He's got no loved ones left in terms of family, and he's utterly lost in life. That makes him vulnerable and willing to believe the woman he once considered his one true love can point him in the right direction like a compass. But the idea of her being a friend could be a weapon meant to exploit Thor's biggest weakness: his heart.
As for why this Jane may not be a hero, it comes down to Mjolnir, which got destroyed by Hela, Odin's daughter. That led to Asgard's destruction, many deaths and Thor eventually getting his Stormbreaker ax forged to become a hero again. Thus, it's fair to wonder how and why the hammer came back into play with this new herald in hand. And the first clue is found in the rules previous films set up about something broken from Asgard needing dark magic to get fixed.
That rule was why Odin and Heimdall didn't overuse dark energy to transport folks to and from the kingdom after Loki helped destroy the Bifrost Bridge. Even Thor acknowledged that was something not to avoid, and given he didn't try to fix the hammer, doing so may come at a high price. So, if Mjolnir was restored and sent to Jane, whichever entity did that could be possessing or mind-controlling her. The MCU can even retcon the power of the All-Back Necrosword to infect her like this, with Gorr being the main candidate to use a corrupted Jane as his avatar to trick Thor. The comics depicted Gorr as a more physical threat, but this would make him a cerebral mastermind and give Thor his toughest test yet.
Another reason Jane could come of ass evil is that misdirection and fake-outs are something the Asgardians, and the MCU on the whole, love doing. Loki pretended to be Odin on the throne, while Hela hid she was Odin's daughter. Thus, another bout of duplicity could see some other rival of Odin's family, maybe even a reborn Hela, taking on Jane's form to either kill or co-opt a broken Thor. That would create another tragic, emotional and unpredictable arc, reminding audiences they can't always trust what they see, and push Thor to his limits.
To see if Jane is who she appears, Thor: Love and Thunder hits theaters July 8.