Spoilers from the supposed leaked teaser, consider the teaser part with caution because it hasn't been confirmed to be official yet.

There were several rumors that she was Shathra and now I'm sure of it, here are the reasons.

1. In the teaser she speaks to Spider-Man in a threatening tone, telling him to stay out of her way. This sounds like a villain's line, and aside from Tombstone, she's the only other character in the teaser to have lines, and Marvel usually puts the main villain of the movie in the teasers.

2. Sadie said in an interview something like "they forgot that hair color can change," in this case she's talking about her character, and Shathra's human form in the comics has black hair. I know she could be any other character, but just to point it out.


3. Of all the characters confirmed in the film, only 3 could be the main villain: Tombstone, Scorpion, and presumably Shathra. I rule out Tombstone because this film doesn't seem to be focused on gang stuff, and judging by the direction things are going, I think he fits better as a secondary villain. Also, Scorpion was never meant to be the main villain, and since "Shathra" has lines in the trailer, and Marvel usually puts scenes or lines from the main villain in the teaser, like in Age of Ultron, if Scorpion were the villain, they would have included him.


4. Her character was confirmed in Secret Wars, and of all the possible characters she could be, Shathra is the one that best fits a multiverse story.

5. Of all the characters she could play, the number has narrowed down with her apparently being confirmed as the villain in the film, meaning it can't be Mayday Parker, Gwen, Spider-Woman, Firestar, or Jean Grey, and much less Mary Jane, leaving either Shathra or her turning into a female chameleon because there was this rumor that the villain of the film would be a shapeshifting villain, but Marvel is trying to be more faithful in recent films like in FF and Thunderbolts, so it can only be Shathra.


I could be wrong, but I'm very confident in this theory.
Theory: I'm confident in this theory that Sadie Sink is Shathra.