Marrow is a child of the 90's bad girl era of comics (http://en.wikipedia.org/wiki/Marrow_(comics)). She was rewarded with a place on the X-Men after killing a bunch of humans (that's not even a joke). A mutant, she has 'uncontrollable bone growth' powers that let her pull bone knives and other fun things out of her body to stab you with. She's currently enjoying a resurgence as a member of the new X-Force.
Getting The Look
One of my favourite things about DCUO is that you can create characters like Marrow that you're probably not going to see in an official Marvel game.
Marrow has had quite a few different looks for a character that hasn't appeared in a huge amount of comics. This version is based on how she looked just after she joined the X-Men (and was prettied up from her original appearances).