"This little girl," said the Scarecrow to the soldier, "wishes to cross the desert. How can she do so?"So Dorothy and her friends set out south from the Emerald City to find Glinda to ask her for help. After facing fighting trees, a village of china people, a giant spider that the Cowardly Lion had to kill, and an encounter with the Hammerheads that caused Dorothy to use the Golden Cap one last time, they finally arrived at Glinda's palace.
"I cannot tell," answered the soldier, "for nobody has ever crossed the desert, unless it is Oz himself."
"Is there no one who can help me?" asked Dorothy earnestly.
"Glinda might," he suggested.
"Who is Glinda?" inquired the Scarecrow.
"The Witch of the South. She is the most powerful of all the Witches, and rules over the Quadlings. Besides, her castle stands on the edge of the desert, so she may know a way to cross it."
"Glinda is a Good Witch, isn't she?" asked the child.
"The Quadlings think she is good," said the soldier, "and she is kind to everyone. I have heard that Glinda is a beautiful woman, who knows how to keep young in spite of the many years she has lived."
She was both beautiful and young to their eyes. Her hair was a rich red in color and fell in flowing ringlets over her shoulders. Her dress was pure white but her eyes were blue, and they looked kindly upon the little girl.Glinda knew the secret of the silver shoes, and told it to Dorothy in exchange for the Golden Cap. According to what she said, she then gave the Golden Cap back to the the king of the Winged Monkeys. (Or, that's what we assume. The Scarecrow says she commands their services later. What this line means, we don't know.)
But more importantly, Marvelous Land makes it clear that Glinda was well around when the Wizard had his reign. She says that she had spies who kept an eye on him, so it's clear that she knew that he wasn't really a Wizard.
The early Oz books make it clear that Glinda is very knowledgeable about magic: not only does she know how the Silver Shoes work, she is also well aware of what would happen to the Magic Belt if Dorothy used it to get home.
The Emerald City of Oz, however, tells us that Glinda has been around for a long time. Perhaps even centuries. Ozma tells this legend:
"It is said that once—long, long ago—a wicked King ruled Oz, and made himself and all his people very miserable and unhappy. So Glinda, the Good Sorceress, placed this fountain here, and the King drank of its water and forgot all his wickedness."It seems that Glinda rarely acts without being bidden or visited. In Wonderful Wizard, Marvelous Land, Ozma of Oz and The Emerald City of Oz, Glinda doesn't get involved until she is asked. But in Emerald City, she does act unbidden in a way: she creates the Barrier of Invisibility before Ozma can ask her to. And in the above quote from the same book, she appears to act unbidden as well.
More famously is how in Tik-Tok of Oz, she discovers Queen Ann and her army setting out to conquer the world and sends them outside of Oz. Now, it can be argued that the creation of the Barrier was because she knew this would be what Ozma wanted, but the creation of the Forbidden Fountain containing the Water of Oblivion and sending Ann and her army out of Oz seem a little odd in nature.
One could argue that Glinda created the Water of Oblivion for the good of the people (though I did once reflect that that could be why we have such sketchy stories about the pre-Wizard history of Oz), but we have to wonder, what if Glinda actually used this to make the King of Oz more agreeable so she could take the Quadling Country? If this was the case, then we seem to have a case of Glinda clearly manipulating what goes on in Oz for her benefit. (Though the Quadlings seem to have no complaints about Glinda.)
In the later Oz books, Glinda does arrive at the Emerald City unannounced and uncalled for, the one springing to mind first being The Cowardly Lion of Oz. But most of the time, her character is consistent with what Baum established.
But in any case, Glinda seems to be quite trustworthy and a great ally, as well as quite friendly to those who ask for her help.
Even if she is just biding her time.