In X-Men: The Last Stand, Jean Grey uses the mutant codename Phoenix.
The film presents Jean as a powerful mutant whose abilities become linked to a destructive second personality. This storyline follows the aftermath of her apparent death at the end of X2, when she uses her telekinetic powers to save the X-Men and their aircraft.
The movie draws loosely from the Dark Phoenix Saga in Marvel Comics, but its version differs substantially from the comics. In the film, the Phoenix is primarily framed as a repressed aspect of Jean’s own power rather than an extraterrestrial cosmic force.
The title Dark Phoenix is associated especially with the 2019 film centered on the same character and storyline. Marvel Girl is another name used by Jean Grey in the comics, while Shadowcat is the codename of Kitty Pryde.