What Marvel's New Mutants Look Like In The Comics
1. Dr. Cecilia Reyes
Now, this character probably has the biggest changes of any other character in the film.
In The Comics
Now, in the comics, technically Doctor Cecilia Reyes was not part of the New Mutants comics. Instead, she was introduced as part of Operation: Zero Tolerance, a storyline where a new version of the Sentinels capable of passing for normal humans almost succeed in defeating the X-Men.
A emergency room doctor raised in the Bronx, the black Puerto Rican mutant got roped into the world of the X-Men for a while. Superhero life was never for her though, and her time was brief, though she recurs as a guest star even to this day.
Reyes had the mutant power of an impenetrable force field that generates around her body in response to force against her, a powerful defensive measure, however towards the end of her stay with the X-Men, she did also start learning she could shape the field and use it offensively too.
In The Movie
As you can see, in the movie, Dr. Cecilia Reyes is not black, which is bound to attract some controversy. Played by Alice Braga, Reyes may still be Latin American in the film and that in fact seems likely, but yeah, there's a definite race change going on here, and that is sure to be a cause for discussion in the months leading up to the film's release.
However, the short teaser trailer does also suggest a number of other changes to the character: for one, while she is not seen to do anything overtly malicious to the teens in her care, there is definitely something sinister about this Reyes. She watches them over security monitors, appears to be injecting them with mysterious green fluids and tells them odd facts about venomous reptiles. It's all a bit...creepy. Plus, she may be holding the teenage mutants against their wills in whatever this haunted house/hospital facility actually is.
Finally, there is also a brief shot that appears to show Reyes covered in blood and deep cuts, perhaps from an out-of-control Wolfsbane, which implies another difference: either she has different mutant powers or she has no mutant powers in the film universe. That, I suppose, remains to be seen.