Number Ninety-Six was a female Skrull Infiltrator who volunteered for a program designed solely for the purpose of creating a "perfect" Mar-Vell Sleeper Agent.
Like the ninety-five volunteers before her, Ninety-Six was deemed a failure due to several genetic and behavioral defects that had been accidentally tailored into her DNA following the experiment.
She was placed into a tech-induced coma and stored inside a secret warehouse filled with the other ninety-five failures for an undetermined amount of time.
A short time after the Skrull Invasion of Earth ended, the X-Men villain known as Mystique stumbled upon the warehouse and after thoroughly studying each of the ninety-six failures, decided to bring Ninety-Six out of her coma.
Because Ninety-Six's real memories had all been overwritten and erased by the copied memories of the true Mar-Vell implanted in her mind by Skrull scientists, Mystique was easily able to manipulate Ninety-Six into killing countless, innocent Church of Hala parishioners by disguising herself as Carol Danvers and speaking sweet nothings into Ninety-Six's ears.
When the real Carol Danvers learned of Ninety-Six's attacks on The Church of Hala and by studying her movements that Ninety-Six was a shape-shifting female, Carol began tracking Mystique down, thinking that Ninety-Six was actually her in disguise only to discover that Ninety-Six was actually Mystique's pawn in a sick and twisted game designed simply to satisfy Mystique's need for "fun".
When Ninety-Six came to the realization that she had been manipulated she lost all sanity and viciously beat Mystique, whom she thought was the real Carol Danvers, into a temporary coma and then mercilessly attacked Ms. Marvel, whom Ninety-Six thought was Mystique masquerading as Carol.
As their fight progressed, Ninety-Six began to overload with radioactive energy born from the genetic defects which had sickened and polluted her body, and began to detonate much like Carol herself had done in the skies above Hong Kong.
With the city of Seattle in imminent danger of being reduced to a smoldering crater due to Ninety-Six's proximity, Carol was forced to absorb Ninety-Six into her body directly upon her detonation.
Thanks to the experiment that turned her into a defective sleeper agent, Ninety-Six gained powers and abilities similar to those wielded by the true Mar-Vell such as super-strength, invulnerability and energy manipulation.
However because of the genetic defects tailored into her DNA, an unknown illness infected her body and somehow polluted her energy-based abilities with a form of cosmic radiation that would cause Ninety-Six's body to detonate like a nuclear reactor and unleash an energy shockwave capable of reducing Seattle and any other city of equal size into a smoldering crater.
While it is presumed that Ninety-Six still possessed her shape-shifting abilities, she was incapable of using them as all of her true memories had been erased by the defective experiment which turned her into a carbon copy of Captain Mar-Vell.