Jean Grey-Summers is a fictional character, a comic book superheroine appearing in comic books published by Marvel Comics. She has been known under the aliases Marvel Girl, Phoenix, and Dark Phoenix and is best known as one of the founding members of the X-Men, for her relationship with Cyclops, and for her central role and transformation in the classic X-Men storyline "The Dark Phoenix Saga".
Jean Grey is a mutant born with telepathic and telekinetic powers. Her powers first manifested when she saw her childhood friend being hit by a car. She is a caring, nurturing figure, but she also must deal with being an Omega-level mutant and the physical manifestation of the cosmic Phoenix Force. She faces death numerous times in the history of the series, the first being when, in her guise as Marvel Girl, she dies and is "reborn" as Phoenix, which in time leads to her second - though not last - death in the classic "Dark Phoenix Saga".
Source: Wikipedia, "Jean Grey", available under the CC-BY-SA License.