Stephen King's The Mist Wiki has a more comprehensive article on the same character. For it, see http://stephenkingsthemist.wikia.com/wiki/Amanda_Dumfries
Amanda Dumfries was a young schoolteacher living in or near Bridgton, Maine. She is a central character in The Mist and its film adaptation, where she is portrayed by Laurie Holden.
She is in the supermarket, purchasing groceries, when the mist sweeps through the parking lot. As the rest of the 70 supermarket patrons panic and grow desperate, Amanda retains her sanity, becoming a friend to young Billy Drayton, who is scared and wants his mother. Since her husband was out of town on business often, he had bought her a Colt .357 Magnum revolver, which is used to kill several monsters inside the mist.
Together with David Drayton and Billy, she fled the store in David's car.