Warning: The following contains spoilers for the Phasma novel.
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The Force Awakens introduced audiences to a new era of Star Wars, including a bevy of new characters of varying allegiances, from Rey to Kylo Ren. There were a few moments of peril and destruction, most notably killing off Han Solo, but outside of a few Resistance pilots (and everyone in the Hosnian System), all of the new characters made it out alive. With everyone set to return for The Last Jedi, almost everyone is also carrying an element of the plot or backstory forward, except for one: Gwendoline Christie's Captain Phasma.
Despite her awesome chrome stormtrooper armor and intimidating demeanor, almost nothing is known about Phasma, partially because she has very little screen time to establish any kind of backstory. That all changes with Phasma by Delilah Dawson, a new novel in the Journey to The Last Jedi series of tie-in stories that tells most of the First Republic Captain's life story leading up to within a few years of The Force Awakens.
While Phasma is fairly thorough, it's important to point out that the story is being told by a questionable narrator - a captured Resistance spy name Vi Moradi who got all of her information from an old friend of Phasma's, Siv. Per Vi's internal monologue, the story is entirely accurate, aside from some slight embellishment, but all her information is second hand from Siv. So, while this bit of Star Wars canon is fairly well documented, there's always a good chance that things didn't play out exactly as described. Even so, it was probably pretty close.
Early Life
The planet Parnassos was once lush and green, teeming with all manner of wild species and ripe for farming - until the arrival of Con Star Mining. Arriving as many as several hundred years before the events of the Star Wars movies, Con Star's Parnassos endeavors were intended to improve the lives of their employees and those that lived on the planet, but some combination of shoddy work and lax regulation resulted in a nuclear explosion at one of their cities, destroying all life within the blast radius and leaving a nuclear fallout that would slowly expand across more of the planet, poisoning the air and killing animal and plant life.
Con Star abandoned Parnassos as all the resources were useless. Some of their employees ended their own lives, leaving behind teams of droids to mindlessly continue running any functional stations. Other inhabitants, whether natives, Con Star employees, or some combination of the two lived on, forming tribes and living in primitive isolation until the memory and understanding of advanced technology was little more than legend and the daily struggle for survival occupied most of their waking lives. This is the Parnassos into which Phasma was born.
Phasma's parents and her brother Keldo lived with a small tribe in a cave they called the Nautilus. When she was a young girl, the already scarce sources of food slowly dried up, wounds - even minor ones - would fester and lead to life-threatening infection, and people's hair began to fall out. Since the Nautilus was one of the few safe havens in the area, other tribes, the Claw and the Scyre, wanted it for themselves. Phasma's tribe would routinely have to fight off attacks from both the Claw and the Scyre, maintaining control of the Nautilus, but slowly decreasing their already small numbers.
Phasma urged her family to unite with the Scyre to form a larger group that could continue to fight off the Claw, but they refused. Eventually, behind their back, Phasma made a secret agreement with the Scyre. One night, she attacked her brother Keldo, stabbing him in the leg and pushing him into the Nautilus, and when he woke up, he and Phasma were the only survivors, and the Scyre had taken over. His leg had to be amputated to avoid infection, but the Scyre had a special oracle salve - harvested from the bodies of their fallen in a sort of pseudo-religious ceremony using devices called "detraxors" - that could be applied to fresh wounds to allow it to heal properly. Keldo and Phasma only needed to pledge their loyalty to the Scyre, which they did.
As a member of the Scyre, Phasma grew into a mighty warrior, both in size - she was one of the tallest people in the Scyre - and skill, regularly leading her own elite warriors in combat against the Claw or on scavenging missions. Keldo, too, became a significant part of the Scyre. While he was missing his leg and not as useful for combat or manual labor, he was one of the few - if not the only - member of the Scyre that could read, and eventually found himself sharing Scyre leadership with Phasma.
Introduction to The First Order
Crashed ships were not uncommon on Parnassos. Some had been abandoned long ago, but other newer ones could also be found thanks to some old defense installments in orbit, still protecting Star Con's former interests on the planet, despite the fact that the corporation hasn't been active there for potentially hundreds of years. One day, Phasma witnessed a glittering ship falling from orbit and dropping an escape pod somewhere beyond Claw territory.
Against Keldo's orders, Phasma races to the location of the crash with some of her soldiers in hope of salvaging something useful, only to find Brendol Hux, the man in charge of the First Order's stormtrooper recruitment and training programs. The Claw arrive first, and Brendol declares that he is seeking the finest warriors in the galaxy and will recruit anyone willing to help him get to his ship, which just so happened to be one of Emperor Palpatine's personal chrome Nabooian yachts.
Phasma pretends to make a truce with Balder so they can both help Brendol, but stabs the Claw leader instead, then her and her soldiers fight off the remaining Claw, telling Brendol he couldn't trust them anyway, and she and her Scyre soldiers set off to help them across the desert to find his ship.
Their journey takes them across several other unfriendly groups, and Phasma proves her fighting prowess each time until even Brendol Hux's stormtroopers begin taking orders from her. She endears herself to Brendol and quickly begins mimicking the former Imperial's accent and posture, watching and adopting the training of the other Stormtroopers over the course of their journey.
When they finally get to the ship, it's in no condition to fly, but they're able to hail the First Order to send a shuttle for pickup. Keldo and the rest of the Scyre intercept them and attack, angry at their betrayal, but Phama, her soldiers, and the First Order wipe out the Scyre, and Phasma even kills her brother herself. Only one of Phasma's original Scyre company, Siv, is still alive at this point. Turning on Siv, Phasma abandons her on Parnassos, leaving to start her life with the First Order, shredding the last vestige Parnassos to begin a new life where nobody but Brendol and a young Scyre girl, Frey, who they will raise as a stormtrooper, knows who she is.
Phasma - First Order Captain
Already a quick learner, Phasma has mastered many of the basics of stormtrooper training - merely by observation - prior to leaving Parnassos behind with Brendol Hux to join the first order. Once officially joining the ranks, her ascent is even faster. Due to a life that required her to be on constant guard, Phasma only sleeps four hours a night, so she spends the rest of her night time hours learning about things she couldn't on Parnassos, like galactic history, military tactics, and technology, including a little "slicing" (hacking).
She gains the title of Captain in less than a year and Brendol Hux grants her half of his stormtrooper training program, a role she now shares with Captain Cardinal, a stormtrooper who wears crimson red stormtrooper armor and designs and conducts the training regiment for the young children being raised to be stormtroopers. Phasma's responsibility is for the older troopers, training them in the actual ways of war and leading them on missions.
After reaching the rank of Captain, Phasma returns to Parnassos and retrieves the chromium plating from the ship Brendol Hux crashed in. Taking it to a former Con Star facility, she has it copy a full set of stormtrooper armor, including a rejected prototype helmet, giving her iconic chrome armor. While on Parnassos, she also retrieves a native beetle who's bite is extremely lethal, but largely unknown of outside of Parnassos.
Scheming with Brendols's son, Armitage Hux, Phasma has the beetle bite Brendol, allowing her and Armitage to fill his shoes. Since Frey had also mysteriously died in a training accident, Phasma believes she has removed the last living person who knows anything about her life before the First Order. Cardinal would eventually uncover part of this plot and attack Phasma, but she kills him as well, taking full control of the First Order's troop training program.
With a new comic detailing more of Phasma's story after The Force Awakens (including how she escaped from the trash compactor), and a more action heavy role in The Last Jedi, Phasma is going to continue to be a major presence in Episode VIII at least - we'll see if she survives until Episode IX. Using the First Order simply as a means to elevate own power, without any regard to any of her allies, she could have an interesting role you play yet.