Unfortunately, the story from The Hollywood Reporter does not mention what new character will be joining The New Mutants or how big of a role this new addition will have.
One of the stars of The New Mutants, Maisie Williams has already spoken about why the delay is a good thing, and if the film is becoming scarier and a new character is being added, perhaps the wait will have been worth it.
The teenage mutants have been a part of the comics since the ’80s and there are plenty of characters in canon that the film could add to the cast.
Word of reshoots often comes with hand-wringing on the part of fans who fear that it is indicative of a problematic film, but often it is just part of the process.