Madison was the sister of Mark, the protagonist of The Kill Order. She is mentioned vaguely as Mark and Trina watch the people in the subway station burn from the sun flares. He instantly thinks of his younger sister, Madison, and their mother like that. He mentions that she was spoiled, but most of it was his doing.
He dearly missed her, but never thought of her much after the incident at the subway tunnels and station. Mark presumed that she and the rest of his family was dead because of their slim survival rate.