Who is Mad Marge in the novel, East Indian?

Mad Marge is a young stowaway who accompanies the boys on the God's Gift as they make the crossing from London to Virginia. Though Marge, as her name suggests, appears to struggle with some form of mental health issue, she also treats the boys (and Sammy in particular) with remarkable kindness, and is a steadfast ally to Tony and his friends when she is not mysteriously missing. Mad Marge ultimately perishes after Ganter abandons her upriver, but she manages to return to the colony to deal Ganter a near-fatal blow to the head before she passes away.