With the arrival of Dark Phoenix in theatres, there are now two X-Men films dedicated solely to the formidable powers of Jean Grey: this film and 2006's X-Men: The Last Stand, both of which are adaptations of the immensely popular "Dark Phoenix Saga" comics storyline. Other than Wolverine, no other X-Men has received quite as much cinematic devotion as Jean Grey, which begs the question: what is so special about this soft-spoken, mild-mannered redhead?
On the surface, Jean doesn't seem like the most obvious contender for "strongest X-Man." In everything from the comics to the animated series to the films, Jean is always portrayed as loyal, gentle, and kind. She also isn't physically imposing, with the Marvel comics database putting her at 5'6" tall, and weighing only 115 lbs. Hardly the type of person you'd think of first when you need a warrior for the front lines of battle.
However, underneath Jean's unimposing persona lies a deep reservoir of power. Jean Grey's mutant abilities include both telepathy and telekinesis, meaning that she can communicate directly with the minds of other people, and that she can move objects using only her thoughts. There are varying degrees of both of these powers in the X-Men film universe — telekinesis was one of Apocalypse's many daunting abilities, and Charles Xavier is a powerful telepath — but the strength and combination of Jean's abilities has always been particularly formidable.
In the original trilogy of X-Men films, Xavier used his own telepathy to repress the bulk of Jean's abilities, fearing that if she knew the full extent of what she was capable of, she'd eventually lose control and harm people. In those films, Xavier's barricading of Jean's powers behind a mental wall resulted in the formation of a destructive dual personality called Phoenix, which eventually took over Jean's consciousness and threatened to tear the world apart. This was a curious (and poorly received) departure from the comics — there, Xavier still creates a barrier between Jean and some of her powers, but he willingly removes it once she's old enough to join the X-Men, and the Phoenix consciousness was a whole separate entity from Jean -- and was the focus of X-Men: The Last Stand, which began with Jean killing her boyfriend, Scott Summers, and ends with Wolverine killing Jean to prevent her from destroying the world.
Thankfully, the events of X-Men: Days of Future Past undid the events of that film, resulting in a world in which Jean was still alive, well, and presumably in full control of her mental faculties. Now, Dark Phoenix is revisiting the same storyline, but this time, in a way that hews at least a little closer to the comics.
In the original "Dark Phoenix Saga," the Phoenix Force is a separate, immortal consciousness that saved Jean from the deadly radiation of a solar flare, joining its powers with her own to create the vastly more powerful Phoenix. Phoenix is then manipulated by the mutant villain Mastermind, resulting in her transformation into the terrifying Dark Phoenix, who battles the X-Men.
In Dark Phoenix, Jean still responds to what the X-Men believe is a solar flare in space, which turns out to be a similar "cosmic force" to the one in the comics. The cosmic force fuses with Jean, imbuing her with tremendous power that manifests in the form of glowing cracks in her skin and sparkling tendrils of light that radiate out of her like stars. This enhanced version of Jean can fly, smash through buildings with a wave of her hand, and crumple trains like they're made of aluminium foil. It's all an extension of her mutant telekinesis power, but magnified to an alarming degree.
This time, the manipulative role of Mastermind is filled by a shape-shifting alien named Vuk (Jessica Chastain), who has been tracking the cosmic force across the universe for years. But the film also still nods to The Last Stand by leaving up the barriers Xavier had placed in Jean's mind as a child, leaving her to discover them for herself. That turns out to be a bad call on Xavier's part, as Jean's feelings of betrayal, in combination with Vuk's prodding, lead to her unleashing her newly heightened powers in destructive and devastating ways.
The central conflict of Dark Phoenix revolves around whether Jean will be able to control her newfound strength, or be destroyed by it. It all boils down to her strength of will, as the film makes it very clear that not one of the X-Men -- or even all of the X-Men -- are any match for her. Which means that, ultimately, Jean's kindness, gentleness, and loyalty may turn out to be her true powers after all.