This says it all, really
Discovered by Herz on one of his crazy Carnival runs, she is apparently the 'mother' of several passengers. Her name is yet to be revealed and her blood makes you invisible to the creepy shit in Carnival. She's also seemingly the only nice person who is part of Elegante canon 8|
After some questioning of the crew, some believe that Mother is most likely the 'system' that brings people back from the dead and everything. You are free to assume that all of Mother's 'children' are the people that are brought back to life on the M.S. Elegante.
Also if you upset her Kage will shank you, so be nice.