Rebecca DeMornay is a charity worker, portrayed by Sonya Eddy. Rebecca appeared twice: in Season Eight’s “The Muffin Tops”, complaining about the stumps left behind the homeless shelter where she works, and in Season Nine’s “The Bookstore”, when she threatens to beat up George Costanza if he doesn’t get his tainted art book out of her thrift store.
It's unclear if she was deliberately named after the popular actress who has the same name.