Jean Grey

Jean Grey is the first student of Professor Charles Xavier. Jean first realized she had powers when her best friend got hit by a car, and Jean could sense her feelings and pain. Ever since then, Jean has been taught how to control her powers. Being the first student of Xavier, she was the last out of the original five to join the X-Men.
Jean first took the codename Marvel Girl and fought along side Scott Summers (Cyclops), her future husband. Jean has left and rejoined the X-Men many times, to get away from the team or just rest.
When the X-Men were in a shuttle-craft, trapped in a radiation storm, Jean was their only hope. The craft was plunging through it straight to earth. The captain couldn't withstand the rays and power coming at him, so Jean absorbed his knowledge of how to fly the shuttle and used her powers to shield herself. Although, Jean was not yet strong enough to withstand the pain, and cried out telepathically.
The Phoenix Force (A fire-incarnet bird shaped being) heard Jean and sensed how much she wanted to save her fellow X-Men. The Phoenix Force duplicated Jean's mind and body, and sent the real Jean in an escape pod down to Earth, at the bottom of Jamaica Bay.
For quite some time the Phoenix force was in the form of Jean. It later turned evil, into the Dark Phoenix. The Dark Phoenix hungered for new emotions and new sensations. It absorbed and destroyed many planets that were inhabited, and turned against the X-Men. The Shiar (an alien race) claimed the Dark Phoenix as a threat, and destroyed the duplicate of Jean.
The X-Men (especially Cyclops) were devastated. They thought Jean was now dead. The Phoenix Force left the dead body and went straight to the real Jean, who was cocooned at the bottom of Jamaica Bay. Jean refused to become one with the Phoenix again.
The mutant named Sinister used Jean's death as an oportunity. He created a biological clone of Jean named Madyline Pryor. Madyline fooled the X-Men into thinking it was Jean. She married Cyclops and had a child, Nathan Christopher Summers (aka Cable).
Later, the real Jean was discovered by the Fantastic Four and reunited with the original X-Men (Cyclops, Angel, Iceman, and Beast) to form a new team called X-Factor. They later joined up with the X-Men again, and Jean took the codename Phoenix. Soon after, Scott and Jean were married. During the events of "Operation Zero Tolerance" Cyclops was injured badly, so Phoenix and Cyclops retired from heroics and lived in their cabin in Anchorage, Alaska.
When Xavier became stressed to the point of tearing the team apart, Storm called Phoenix and Cyclops to come help. They decided they couldn't help him, and formed a new X-Men team called The Astonishing X-Men. When Wolverine was thought to be dead, the X-Men banded together once again and battled the threat, Apocalypse. It was revealed that Phoenix was a member of "The Twleve", a collection of 12 mutants who must destroy Apocalypse.
When Apocalypse was going to use Nate Grey as the host body for his eternal essence, Cyclops saved him and took his place. Apocalypse and Cyclops have now become one, and Cyclops is presumed dead. Phoenix, grieving the death of her husband, left the X-Men.
She has since returned and is among the X-Men group led by Gambit. A strange power-connection is starting to occur between Phoenix and fellow teammate, Psylocke. Phoenix has lost her telekinesis ability and is now using the Phoenix force in new ways, while Psylocke has gainded the power of telekinesis.