I believe the Rain King is first mentioned in Tik-Tok of Oz, in which we’re told, “The Rain King got too much water in his basin and spilled some over the brim. That made it rain in a certain part of the country—a real hard shower, for a time—and sent the Rainbow scampering to the place to show the gorgeous colors of his glorious bow as soon as the mist of rain had passed and the sky was clear.” Polychrome, the Daughter of the Rainbow, is described as the Rain King’s niece, making the Rain King quite possibly (although not necessarily) the brother of the Rainbow, whatever that means for elemental fairy folk. He’s mentioned only briefly in other canonical Oz books, but I have to wonder if he’s working behind the scenes at times. For instance, Kabumpo comes across rainstorms on the edge of the Deadly Desert in both Purple Prince and Silver Princess, and both times they help him cross the desert. Since you wouldn’t expect much rain near a desert, is the Rain King helping out the elephant? How much control does the King really have over when and where it rains? In Gina Wickwar’s Hidden Prince, we finally actually see the Rain King. Well, sort of. He’s described as a large black cloud with a thunderous voice, who lets some sunshine through when he’s in a good mood. We also learn that he has several leprechauns in his employ.
Another thing I have to wonder is whether Fanny the Weather Witch, who appears in John R. Neill’s Runaway, is an ally or competitor to the Rain King. While this witch’s main job is creating wind with her windmill, we also see her producing rain, snow, and hail. She stores these in large tanks, releases them by way of valves, and blows them to places where they’re needed. Since her method of weather production is so mechanized, I can see her taking some business away from the traditional weather-makers in the way machinery replaces people in real-world industry. I don’t know whether this was Neill’s intention, however, and I don’t think he ever mentioned the Rain King in his books.