When Ozma restricted the practice of magic to only a few trusted individuals, there were notably some who ignored her ruling. However, one Munchkin named Bini Aru decided that even though he'd discovered a powerful charm of transformation, he would obey Ozma's word and stop using it and any other magic. However, he decided to record the magic word "pyrzqxgl" on the underside of a floorboard, just in case he might be permitted to use it again.
Well, Bini and his wife Mopsi happened to have a son named Kiki, and one day when his parents were away, Kiki happened to find that floorboard and discovered the secret it held. Being a cross and disagreeable Munchkin boy who wanted to see the world outside the settlement on top of the steep Mount Munch, Kiki quickly realized he could achieve his much-desired goal of seeing the world outside of his mountain home. Very soon, he memorized and mastered the magic word and flew off over the Deadly Desert to the Lands beyond Oz.
Kiki ventured through Hiland and Loland, then flew over Merryland, toured Noland and Ix, and finally arrived in Ev, where he wound up stealing money to pay for dinner and lodging. This caused him to be noticed by Ruggedo, the former Nome King. Discovering that the boy knew how to work transformations, Ruggedo quickly made him an ally and convinced him to help him return to Oz with intent to conquer.
Upon arriving in Oz, Kiki quickly realized that he was nothing more than a means to an end for Ruggedo's plot to make the animals of the Forest of Gugu revolt against Ozma and the human/humanoid people of Oz. Yet he kept dutifully performing transformations as he expected a reward eventually. In the shape of monkeys with lion's heads with eagle's wings and donkey's tails with gold balls at the end (these were dubbed Li-Mon-Eags), they nearly convinced King Gugu to help them attack Oz.
Until the Wizard and Dorothy happened to show up in the first. Kiki wound up panicking and made some hasty transformations, incapacitating everyone who seemed to be in his way, including turning Ruggedo into a goose and the Wizard into a fox.
Ruggedo managed to get Kiki to transform him back into a Li-Mon-Eag, but now the Nome was trying to discover the magic word for himself. However, before he could master it, the Wizard happened to be hiding in the same hollow tree that Kiki would say the word into, attempting to keep it a secret. After the Wizard managed to get the pronunciation down, he turned Kiki into a harmless mute hickory nut.
After a little while, Ozma had the Wizard restore Kiki to his true form, adding that he would be thirsty so he'd drink of the Water of Oblivion. He was unable to tell Ozma anything before he took a drink. They could tell from his clothes that he was a Munchkin, so Ozma decided to keep him in the Emerald City to give him a fresh, good start. And since Kiki's wickedness was based around being ignored, this should have proved to be an improvement in his character.
Except Kiki Aru is never mentioned again in the Famous Forty Oz books.
Kiki sticks out from Baum's other villains because he is not a wicked sorcerer, he is not an elemental being like the Nome King. He is a simple Munchkin boy. He doesn't like to play with neighbors or other Munchkins, but really, Kiki could be any child. He didn't even want to harm the Oz people. His first real crime—theft—was done from necessity. Perhaps if he had never met Ruggedo, he might have lived an odd but happy life outside of Oz. But after being taken in by the Nome and causing problems in Oz, one could see why Ozma needed to make him forget the magic word.
There is a story in Oziana titled "Much Ado About Kiki Aru" in which Bini Aru finds and claims his son. While I liked that part, if also explains why the Wizard never uses the magic word "pyrzxqxgl" again. While I admit that the Wizard knowing this word makes later stories less problematic (why didn't the Wizard transform the sunken city's support beams in Glinda of Oz?), I've found it fun to think that the word still works and the Wizard is just about the only one who knows it, and he uses this knowledge very sparingly.
Whether or not "Much Ado" is canon or not, I think it's safe to say that Kiki was eventually reunited with his parents. If Dorothy related the story to Mr. Baum, who wrote it as a story, then how did she know Kiki's name?