Like the main protagonist of Neil Gaiman's Neverwhere, Richard Mayhew, Anesthesia once lived in our world.  When she "slipped through the cracks" to London Below, she found a home with the Rat-Speakers, people who eat cats and serve the rat population of London, doing whatever the rodents can't do themselves.  Anesthesia helps Richard Mayhew find his way to the floating market, but is taken by the Night's Bridge.