Who is Sister "Creep" from Swan Song and what is their importance?

Sister begins the novel as "Sister Creep," a crazed bag lady in New York City who thinks that Jesus is going to come down on a spaceship from Jupiter and save her during the Rapture. Sister Creep is obsessed with avoiding a buried memory of her past, and even hides in abandoned New York City subway tunnels in her attempt to escape. She survives the blasts because of the tunnels and encounters the man who likes movies when she emerges: her first brush with true evil. She escapes with the image of a crucifix burned onto her neck.