Preparing for X-Men: Dark Phoenix? Brush up on your Jean Grey history--but get ready, it's a doozy. For being one of the X-Men’s most notoriously complicated mutants, Jean's story actually has a relatively simple start. She's one of the original members of the X-Men team, introduced in the very first issue in 1963. She was the token girl on the otherwise all-male team back in the day, and the only mutant with telekinetic powers in the mix--the others, Iceman, Psyclops, Beast, and Angel all had largely physical powers. For years, this was as tricky as Jean got. She was known as Marvel Girl, and she could move things with her mind--no fuss, no frills.
The simplicity didn't last, however. Over time, more of Jean's origin was revealed to be a somewhat insidious effort by Charles Xavier, who had clocked her as an up-and-coming telepath with potentially dangerous abilities. As she was enrolled in Xavier's Academy For Gifted Youngsters--and from there, things were off to the races--the days of simple, easy-to-understand Jean were over.
Over the next thirty-plus years, Jean went on a journey full of retcons, revisions, and reimaginings that elevated her from one of the X-Men's most basic members to one of their most powerful and complicated. Jean's transformation from humble psychic to Omega-level cosmic deity was thanks to the Phoenix Force, a celestial energy housing the power of all life and creation in the universe, which she unwittingly welcomed into her body during a space-based mission gone awry.
Reborn as Phoenix, Jean promptly succumbed to the manipulations of the Hellfire Club, who enticed the Phoenix Force to give into human emotions like rage and desire. Completely out of control, Jean became the Dark Phoenix, a supervillain who posed more danger to the universe than Galactus himself.