The daughter of Scott Summers and Jean Grey in an alternate timeline, young Rachel Summers inherited her mother's potential for telepathic and telekinetic powers she possessed as Marvel Girl of the X-Men, a team of mutant adventurers fighting for peaceful coexistence between mutants and humans.
In the mainstream reality, the X-Men prevented an attempted assassination of anti-mutant Senator Robert Kelly. However, in Rachel's timeline, Kelly was killed. The assassination caused intense widespread public paranoia against mutants, and, in the next presidential election, an anti-mutant candidate won. His administration unleashed giant mutant-hunting robot Sentinels to eliminate mutantkind, and the Sentinels decided that the best way to do so was to take over the United States. Over the subsequent years they killed or imprisoned virtually all known superhuman beings within North America.
One day, whilst Rachel was still a child, the X-Men's base of operations at Professor Charles Xavier's School for Gifted Youngsters was attacked and demolished by federal troops. The only survivor at the school was Rachel, whom the soldiers found by the side of Xavier's body. They used drugs to neutralize her developing psionic powers before she was tortured and brainwashed in the laboratory of the sadistic Ahab. Following extensive behavior modification, Rachel became a mutant "hound," using her psionic powers to hunt down other mutants for the authorities. Rachel's superiors branded her face with a grotesque pattern of tattoos to mark her as a hound, and forced her to wear a leash. Rachel occasionally killed mutants herself, and it is known that some of those were people she loved. Rachel's grief and shame at what she was forced to do as a hound, and her anguish over the deaths of her parents and friends deeply scarred her soul, leaving her bitter and intolerant toward humans who persecute mutants.
Eventually, Rachel was placed in the South Bronx Mutant Containment Facility where she rejoined the surviving members of the X-Men in a plan to end the Sentinels' reign by changing history. Rachel used her psionic powers to exchange the consciousness and spirit of the adult X-Man Kate Pryde with that of Pryde's teenage self as she existed at a time just before the attempt on Kelly's life. It was hoped that the adult Kate, in her younger self's body at that earlier time, could warn the X-Men about the assassination attempt so they could stop it, thereby preventing the chain of events from occurring that led to the Sentinels' rule. However, history cannot be changed in this manner. Rachel, as it turned out, actually caused Kate to exchange psyches not with her younger self in the past of her own reality, but in the mainstream reality. Kate succeeded in preventing Kelly's assassination in the mainstream reality, after which the Pryde's psyches returned to their proper times and realities. On her return, Kate found that other X-Men, except for Rachel, had all died in an attack on the Sentinels' Manhattan headquarters.
Kate still did not give up her vain hope of altering her reality's history. She hypnotized Rachel into using her psionic powers to tap into the Phoenix Force and send herself back in time when Kate said the words "Dark Phoenix". Thus Kate hoped that Rachel herself could change history. Together, Kate and Rachel illegally entered the laboratory of Project Nimrod, which was the attempt to construct the most highly advanced Sentinel possible with the technology of that time, in the hopes of sabotaging the project. Pursued by guards, Kate and Rachel locked themselves within the laboratory, whose walls were proofed against Kate's mutant power to phase through solid matter. Rachel believed herself and Kate to be trapped, but Kate said the words that caused Rachel to project herself back through time. Project Nimrod's creation, the Sentinel called Nimrod, then killed Kate and travelled back in time to find Rachel.
Rachel found herself physically transported, body and mind, to a time shortly before the present. But, like Kate's spirit, Rachel had traveled not to the past of her reality, but to the past of the mainstream reality. She went to Xavier's mansion, but could not bring herself to stay on realizing that she had come back to the wrong reality, and fled without identifying herself. Pursued by the psychic vampire Selene, Rachel was rescued by the X-Men. After Xavier read her mind, she was invited to join the team.
Rachel was shocked to learn of the supposed "death" of Jean Grey in this reality. Visiting the home of Grey's parents while they were absent, Rachel found a holo-empathic crystal that was imbued with an imprint of Grey's personality. Unaware that Jean had not actually been Phoenix, Rachel refused to believe that Jean could be evil, as people claimed Phoenix was, and she decided to claim the name and power of Phoenix as her own birthright. Through her own actions, Rachel sought to redeem the name Phoenix in this reality. Rachel also believed that as the new Phoenix she could use her power to help her fellow mutants, and thereby atone for what she had done as a hound in her own reality. It was at this moment while holding the crystal that Rachel first manifested about her the bird-shaped aura of energy associated with the previous Phoenix. From that time on, Rachel was able to draw to a limited extent upon the vast power of the cosmic entity known as the Phoenix Force. The alien being known as the Beyonder once hinted that Rachel could potentially become equal in power to the previous Phoenix. Indeed, the Beyonder once granted Rachel the full power that the previous Phoenix had possessed, but he reclaimed it shortly thereafter.
Soon after, Rachel's teammate Wolverine severely injured her to prevent her from vengefully murdering Selene. Fleeing to heal herself, she was fooled by the six-armed sorceress named Spiral and taken to the alternate dimension of Mojoworld where she was held prisoner for several months. Rachel eventually escaped to London where she was tracked by agents from Mojoworld. With the help of Kitty Pryde, Lockheed, Nightcrawler, and Captain Britain, Rachel defeated her would-be captors and together they formed the British super-team Excalibur.
After many adventures with Excalibur, Rachel finally managed to return to the future she had come from. While she couldn't change her past, she and her teammates were able to change the directives of all the Sentinels of the era to preserve all life, thereby ending the genocide that had prevailed for years. On the return journey, Captain Britain was lost in the timestream and it was discovered that Rachel had to exchange places with him for him to return. She accepted, and was carried to another future time that was ruled by the eternal mutant Apocalypse. There, Rachel founded the Clan Askani to help in the struggle against Apocalypse's tyrannical reign.
As the Mother Askani, Rachel was responsible for transporting her parents to the future to care for Cyclops' infant son Nathan, who had been taken to the future in the hopes of curing a techno-organic virus that Apocalypse had infected him within the past. The Clan Askani saw in the infant the potential to be a great warrior, and trained him to become the soldier known as Cable. The effort of transporting the two X-Men exhausted Rachel's frail old body and her physical form died, leaving her to live on in an astral form.
Sometime later, Cable journeyed to the end of time and found a youthful Rachel being held prisoner by Gaunt, a man who had been imprisoned there for his crimes. Cable challenged Gaunt to a duel to decide the fate of Rachel and emerged victorious, returning Rachel to the present with him. With her own timeline erased and having been abandoned by the Phoenix Force, Rachel decided to retire from the costumed adventuring life and began attending college. Soon after, Rachel became embroiled in the Dark Sisterhood's attempted takeover of the United States. Rachel aided Cable in defeating the Sisterhood and their leader, Finality, after which she returned to college.
Months later, Rachel was captured by the mutant predator Elias Bogan who used her as his pet telepath, enslaving other mutants to do his bidding. After Bogan's defeat at the hands of the X-treme, X-Men team, Rachel was freed from his telepathic control and rejoined the X-Men, taking the codename Marvel Girl to honor the memory of her mother. She has also changed her last name to "Grey" becoming Rachel Grey possibly to express disapproval at her father's continuing relationship with Emma Frost, though she and Emma have made a truce of sorts during one of the team's missions in Hong Kong. After House of M and Decimation, where most of the world's mutants lost their powers, the government had Sentinels instituted at the X-Mansion to protect the mutants in case any enemies used this low point to attack. Though their intentions were good this time, it reminded Rachel too much of the previous timeline when Sentinels herded mutants into concentration camps.
Rachel spent some time with her grandparents, bonding with her grandfather. At a family reunion with all her relatives, a commando unit under the order of the Shi'ar attacked the party, killing everyone including Rachel's grandparents, in hopes of wiping out the Grey genome. Though unable to kill her, they were able to graft a deathmark on her back that would allow them to find her wherever she went. Afterward, at the graves of the Grey family, Rachel vowed a terrible vengeance on the Shi'ar.
Even after swearing a vow of vengeance on the Shi'ar Professor Xavier recruited Rachel to come on his mission to stop her uncle Vulcan from destroying the Shi'ar, Since the event that revealed Vulcan was Rachel's uncle, Professor X spent much time with Rachel in order to teach her to hone her skills as a telepath and improve them since Professor X had lost his powers. Professor X recruited Rachel along with her uncle Havok, Nightcrawler, Warpath, Darwin and Polaris in a space mission to stop her paternal uncle Vulcan from destroying the Shi'ar empire. Xavier, who recently was stripped of his powers, recruited Rachel to serve as his telepathic "eyes and ears" during their mission. Aware of Rachel's vendetta against the Shi'ar, Xavier has agreed to use their trip into space to find out who in the Shi'ar Empire gave the order to wipe out all members of the Grey family and has warned Rachel that they will deal with the people responsible for her recent losses Xavier's way.
While in space, the team was attacked by Korvus, a Shi'ar warrior sent to kill Rachel. Korvus's ancestor Rook'shir was a previous host of the Phoenix Force, and a small portion of the Phoenix's power was left behind in his sword, the Blade of the Phoenix. With this power, Korvus made short work of the other X-Men, but when Rachel blocked the sword their minds were involuntarily linked and Rachel learned that Korvus's family was also murdered by the Shi'ar government because of their connection to the Phoenix. Part of the Phoenix power from the sword was transferred to Rachel. Rachel claimed that rather than having taken the power, the power chose to go to her. When this happened, Rachel's normally gold energy aura turned blue, the same color as the Blade of the Phoenix. She then telekinetically disabled an explosive implant that the Shi'ar chancellor K'tor was using to force Korvus's obedience.
Due to her connecting with the sword, Rachel also connected to Korvus and the two began to share memories. Rachel discovers the Phoenix Force formerly in the blade is just an echo of the Force. This blue shadow of the Phoenix soon began influencing Rachel's behavior, causing her to design a darker new uniform and begin a romance with Korvus. Not long after these events, Vulcan took the life of his father Christopher Summers, both Rachel and Korvus joined the Starjammers, to stop the evil Vulcan.