The Dust Witch will turn out to be the most significant and dangerous member of the carnival, apart from Cooger and Dark. The boys will get a better look at her when they go to check out the carnival in chapter 24, on the day after its middle-of-the-night arrival; and she will come searching for Jim's house in the balloon in chapter 29.
Her most significant appearances will be in chapter 43 when she comes to the library to fetch the boys back to the carnival, and in chapter 47 when she participates (unwillingly, for once) in "The World Famous Bullet Trick."
Witches play a significant role not only in Bradbury's work -- a family of witches appears in the stories from his first book, Dark Carnival -- but even in his ancestry. As detailed by biographer Sam Weller in The Bradbury Chronicles, Mary Perkins Bradbury, the wife of ancestor Thomas Bradbury, who emigrated to the colonies from England in 1634, was charged and convicted of witchcraft during the Salem Witch Trials in the Massachusetts Bay Colony in 1692, when she was 72.
Though found guilty and sentenced to die along with 19 others who died by hanging, she somehow escaped the gallows (the guards were bribed to set her free, Weller suggests), and died a free woman in 1700. Bradbury has always been keenly aware of unfair social and political persecution of innocents, based on the experience of his forebears.