Who is Sylvia Katzenhide, Cluthra, Amberina and Ivrene, Mrs. McGuff from The Cat Who Turned on and Off and what is their importance?

Sylvia Katzenhide, Cluthra, Amberina and Ivrene, and Mrs. McGuff are dealers. Unlike Ben Nicholas and Hollis Prantz, they are essentially who they say they are. Their essential function is to provide information to Qwilleran. They point to the truth that he uses to break through the illusions projected by Glanz, Duckworth, Nicholas, and others with a vested interest in preserving them.