Marrow is a mutant who, due to the physical appearance of her mutant abilities, hid herself as a young girl in the sewers underneath New York City as a member of the Morlocks, a group of mutants who were also physically deformed. After befriending Iceman, Marrow joined the mutant team know as the X-Men.

noun

the soft, vascular [having vessels or ducts, especially for moving blood], fatty tissue that fills the cavities of bones

Iceman
mutant