Marrow (Sarah), is a Marvel Comics character, associated with the X-Men. She is a mutant whose bones grow out of her skin and can be removed from her body, providing her with potential knives and clubs as well as body armor.
As a child, Marrow was taken in by the Morlocks, a band of grotesque-looking mutants who hid in tunnels beneath New York City. As a young adult, she formed the violent splinter cell Gene Nation until, under the orders of Morlock leader Callisto, she joined the X-Men to redeem herself. She made progress controlling her powers and learning a moral code, but eventually fell in with the paramilitary group Weapon X.
She has shown herself to be a staunch team player (more-so with the X-Men than with either Gene Nation or Weapon X) who is prepared to risk her own life to protect those of her teammates. Despite her apparent disdain for humanity, she has revealed that she would like nothing more than to be able to fit in with them and be happy.
While there have been occasions that this has happened for her (in an art gallery with Colossus and immediately following her genetic redesign by Weapon X), they have never lasted long and only served to increase her anger and bitterness towards the world when they have been taken away from her.
Marrow is a mutant with the ability to enhance the growth of her skeletal structure. Initially this was uncontrollable, but after enhancement by a Skrull medical facility, and later by Weapon X, she can mostly control this, despite each enhancement having partially failed over time. She utilizes this power in many ways, including the creation of knuckle guards, spears, and projectile spikes. She also possesses a healing factor and enhanced immune system, much in the same vein as Wolverine since every time a bone is ripped out a wound remains which closes itself soon after. She also possessed two hearts to compensate for her random bone growth, so when Storm ripped one out she was able to survive. It's possible that the first heart could have regrown due to the healing factor.