Marrow was one of several new characters and concepts that came to prominence during the meteoric rise in the already immense popularity of the X-Men during the '90s. Other new hit characters included cyborg mutant from the future Cable, the mercenary Deadpool, and the teleporting Blink. Marrow's trajectory, however, didn't quite match theirs.
Covered in bony spikes and a violent persona, she fit right in with the mindset of the decade. This saw Marrow show up in several comic books and even video games. Unfortunately, Marrow's popularity quickly mutated into obscurity, with a series of horrific events coinciding with her eventually no longer being a part of the X-Men comic books. Here's the bone-crushing story behind the former Morlock and how she might just return to prominence in the modern comics.
Sarah, who would later become known as Marrow, was a child when Mr. Sinister's "Mutant Massacre" nearly killed all of the Morlocks. She and other Morlocks were taken away to another dimension by Mikhail Rasputin. This harsh reality had a dog-eat-dog mindset, which toughened the young Sarah into a dedicated fighter. Likewise, she also looked up to Morlock leader Callisto as a motherly figure. During this time, her mutant ability to control and even weaponize her bones. This allows her to excrete said bones as a protective carapace or as weapons. This painful process is leveled out by her enhanced healing factor and other physical attributes. When the Morlocks were brought back to Earth, a few years had passed, with Sarah now a teenager/young adult.
She and the Morlocks, wanting to never be victimized by genocide or prejudice again, lashed out at and orchestrated terrorist attacks against human society. They redubbed themselves Gene Nation and embarked on several violent acts. This brought them in conflict with the X-Men, who Marrow would be forced to later team up with when Callisto was injured by a Sentinel. Callisto also tried to soften her spiritual daughter's growing rage, and Marrow ironically took the place of Shadowcat and Jubilee when she was mentored by Wolverine. Marrow's time with the X-Men helped her gain many new friends and a newfound control of her powers, the latter of which she used to soften her physical appearance. She not only tried to work with potential rival Jubilee to find the missing Wolverine, but also consoled Colossus using his own artwork. Unfortunately, these good times wouldn't last, and Marrow's time in the sun would soon come to an end.
In 2000, Marrow was one of several X-Men who were playable in the fighting game crossover Marvel vs. Capcom 2: New Age of Heroes. This reflected her huge prominence in the comics the years beforehand, but that same year, this streak of relevance would come crashing to a halt. Marrow was written as having left the X-Men during the failure that was the "Revolution" relaunch, and her appearances from then on were intermittent. Her being essentially replaced by the very similar original character Spyke in the X-Men: Evolution cartoon also kept her from gaining more mainstream exposure. She was briefly a member of a new Weapon X until she discovers that they were still in the business of using and experimenting on mutants. From there, she would try to reform the Gene Nation from their terrorist ways.
Following M-Day, Marrow and several other mutants would lose their abilities. Her appearance was still somewhat unchanged, however, leaving her unable to truly live as a human, either. She would join the terrorist group X-Cell, which was made up of former mutants who attacked the government in the belief that they were behind the loss of their powers. Eventually joining a new version of X-Force, Marrow now seemingly had her powers back. This was revealed as happening due to Marrow accepting a procedure from an individual named Volga, with the restoration of her powers killing the baby that she was pregnant with. This had a traumatic effect on the young woman, and Cable chose to use a device to suppress her memories of the horrifying event.
This increasingly sad series of events in Marrow's life may finally have a happier resolution, however. She's been shown as one of the several mutants who have accepted Charles Xavier's invitation to the mutant paradise of Krakoa. Her powers are now likely fully restored, and the idyllic nature of the mutant island is a far cry from the Morlock sewers that a young Sarah grew up in. Hopefully, she'll take a temporary break from this newfound respite and rejoin the X-Men whenever she has a bone or two to pick with their enemies.