Pompus is the reigning king of Pumperdink.
A truth-telling golden mirror once described Pompus as a "Fat Old King." Pompus is somewhat authoritarian and mercurial, without being overtly evil or truly oppressive. He has a bad habit of sentencing his subjects to be dipped in a purple inkwell for minor and trivial offenses; but the dipping is a relatively mild punishment.
Pompus is married to Queen Pozy; they are the parents of Prince Pompadore. (Kabumpo in Oz)
Pompus later passed judgment on Randy for eating the royal grapes and calling them sour. However, Randy helped to save the lives of Pompus and his family when they were kidnapped and transformed by the wicked princess Faleero, who sought to install Pompus' evil brother Kettywig as a puppet monarch. (The Purple Prince of Oz)
Pompus also appears in the non-canon Oz novel The Blue Emperor of Oz.