Marrow is an on-and-off villain from the X-Men franchise who was originally an antagonist but became more of a neutral character as the series progressed, though she would continue to have a stormy relationship with many members of the group, most notably Storm, who she had almost killed in the past during a conflict.
Marrow's main opposition to the team came from the fact she was a prominent member of the Morlocks, a gang of mutants who were outcast from normal society.
Unlike the X-Men, the Morlocks tended towards a neutral approach and would occasionally lash out at the world, Marrow being a prime example of one of the more aggressive members of that faction.
Wolverine and The X-Men
Marrow appears as a minor antagonist in Wolverine and The X-Men, though her reasons are tragic and she regrets her actions.