Dolores Umbridge introduces a seemingly endless string of problems to the students at Hogwarts. She serves as the Senior Undersecretary to Cornelius Fudge, the Minister of Magic, and is extremely loyal to him. Like Fudge, she refuses to acknowledge the return of Lord Voldemort, and she believes that both Dumbledore and Harry Potter are devious liars. Fudge forces Dumbledore to appoint Umbridge as the new Defense Against the Dark Arts teacher, and from her first moment at the school, when she interrupts Dumbledore's welcome speech to give a long, tedious lecture about her role at Hogwarts, she is massively disliked by both students and faculty. This dislike, however, is rooted in more than just her unpleasant personality. Defense Against the Dark Arts is a crucial class at Hogwarts, but Umbridge refuses to teach her students any actual Defense spells, instructing them instead to simply read their textbooks during class time. Her students are forced to meet in secret. The only students who seem to respond favorably to Umbridge's presence are Draco Malfoy and his gang of Slytherins, and even then only because Umbridge expresses such a strong dislike for Harry Potter, whom Draco has long despised.

When Umbridge is appointed High Inquisitor of Hogwarts, her negative effect on the school increases a hundredfold, especially for Harry. She insists on posting new and progressively more ridiculous Educational Decrees to the student bulletin boards. She bans Harry from the Quidditch team and forces him to carve “I must not tell lies” into the back of his hand until he bleeds. She reads Harry's mail and prevents him from corresponding with Sirius. Ultimately, she discovers the secret meeting place of her Dark Arts students and forces Dumbledore to resign, effectively destroying the central spirit of Hogwarts. Umbridge's name is very close to the word “umbrage,” which means “to take offense,” and this characterization fits Umbridge perfectly. Umbridge takes much offense at Harry and even more at the way Hogwarts is being run: besides forcing Dumbledore's resignation, she fires Professor Trelawney and Hagrid and clashes frequently with Professors McGonagall and Snape. Umbridge ultimately gets her comeuppance, but the damage she causes will surely linger even after her very welcomed departure from the school.