"Lord Voldemort has seemed to grow less human with the passing years, and the transformation he has undergone seemed to me to be only explicable if his soul was mutilated beyond the realms of what we might call 'usual evil'"
, in Harry Potter and the Half-Blood Prince
Dolores Umbridge is introduced in the Harry Potter and the Order of the Phoenix, as the replacement for the perpetually short lived job that is Defense Against the Dark Arts teacher. She is described as a short, squat woman with a fondness for the color pink, kittens and a girlish way of talking, who is also an arrogant bully full of contempt and a sadistic streak. Before becoming Defense Against the Dark Arts teacher, she was an official at the Minister of Magic, in a position ("senior undersecretary") that seems to have mostly consisted of sycophancy. At Hogwarts, she attempts to teach Defense Against the Dark Arts out of a textbook, and claims that her students would not need any actual practice. This is in part because she is staying true to the Ministry of Magic's party line that Lord Voldemort has not returned and there is nothing to fear. Over the course of the fifth book, she manages to seize control of Hogwarts, but by the end of the book she has been driven from power, and from sanity. She is also revealed to be more than just a boring teacher, a bully or a bureaucrat, doing many things that cross the line into actual sadism.
She returns in the seventh book, and here her darker nature is shown even clearly. After Lord Voldemort secretly takes control of the Ministry of Magic, Dolores Umbridge is one of the people who implements his plans to investigate the "purity" of wizards and witches. Although it is not clear if she knows who she is working for, her relish at interrogating and manipulating is obvious enough. In some ways, her lack of ideological interest makes her even worse: she is interested in bullying simply for its own sake. Her backstory, and how she got to be the evil person that she is, are not explored in the books, but again, that makes her all the more scary. This is why I chose the quote above to begin this entry. Much of the darker parts of Harry Potter explore the depths of evil that Lord Voldemort goes to in his quest to cheat death, and are a portrayal of what happens when people go beyond the "usual evil". Dolores Umbridge, on the contrary, is a perfect portrait of the "usual evil". She doesn't have any grand plans or causes, or even any self-awareness of why she is intent on harm. She is merely a person who has a penchant for causing harm and controlling others, and will go along with whoever or whatever is in power to give reign to it.