|“||The thing is, my green girlie, it is not for a girl, or a student, or a citizen to assess what is wrong. This is the job of leaders, and why we exist.||”|
|— The Wizard to Elphaba Thropp, Wicked: The Life and Times of the Wicked Witch of the West|
Maguire uses elements of L. Frank Baum's Wizard from the original 1900 Oz book The Wonderful Wizard of Oz, to make the Wizard use many different illusions to fool his audience into thinking he really is a force to be reckoned with and the most powerful living source in all of Oz.
The Wizard in Wicked is undeniably a very cold blooded individual, with a selfish spirit and a superficial personality. He has no regard for human feelings and behind closed doors comes off as very sociopathic, if simply not just a flat out sociopath. He can be charming, but falsely so, and finds that the Ozians are a very gullible folk compared to people from his world (our world), and he uses that to control, manipulate and trick them in every way he can. Bottom line; in the novel Maguire portrays the Wizard as a very bad, power hungry man, an Adolf Hitler of sorts.
When Oscar Diggs first arrived in Oz before Elphaba Thropp was born, he came to the land via a hot-air balloon originally seeking the Grimmerie. While in Oz he made a living as a con man and a hustler who happened onto a world where he could literally make himself into a king overnight.
After he used his fake illusions and false magic to overthrow the Ozma Regent, Pastorius, Oscar had him assassinated. He then kidnapped Pastorius's baby daughter, the Ozma Tippetarius, and gave her to La Mombey Impeccata, a witch from Gillikin to keep and hide her away forever. The witch transformed the princess into a kitchen boy she would later call "Tip" and use as her personal servant. Since there was no heir to the throne, Oscar proclaimed himself the new official ruler and made everyone call him "Wizard" instead of "King". Shortly after he ascended to the throne, he ordered the construction of the yellow brick road. He took up residence in the Royal Palace in the centre of the city and slowly brainwashed many of his subjects. He ruled over the Emerald City with an iron fist while secretly requiring people with true magical talent such as Madame Morrible and La Mombey to cast all sorts of spells for him.
It is revealed that the Wizard is indeed behind some of the most horrific and disastrous events in the story, with one of his accomplices being Madame Morrible. Glinda Upland and Elphaba Thropp met the Wizard when they were still students at Oz's Shiz University, but when they spoke to him they both realized that the Wizard wasn't so "Wonderful" after all.
The Wizard is later revealed to be the illegitimate father of Elphaba Thropp, having conceived her with Melena Thropp decades before the former's deluded power trip and Dorothy Gale's unexpected arrival. Melena was filled with promiscuity, this resulted in the Wizard seducing her with a mysterious green beverage known as the "Green Miracle Elixir". They had a one night stand before he became the ruler of Oz and Melena became pregnant while her religious husband Frexspar was away.
It is also hinted that the elixar Melena was given was the reason why Elphaba's skin is green, but Melena did not know this because people from the world where the Wizard and Dorothy come from cannot make out Oz's unique writing system. Vise-Versa.
It is also under the Wizard's direction that the Animals of Oz — most notably the Goat teacher from Shiz University, Doctor Dillamond — are placed under strict control. This cruelty causes the final split between Elphaba and the Wizard, leading to her transformation into the Wicked Witch of the West.
In the end of the novel, it is hinted that the Wizard may have committed suicide after he returned to Kansas, leaving his empire in Oz behind.