Bellatrix LeStrange is a Death Eater, first introduced in Harry Potter and the Order of the Phoenix as a minor character, and then gaining importance in the next two books. She is a Death Eater, and in the last book she is portrayed as perhaps Lord Voldemort's most trusted deputy.
Bellatrix LeStrange is a member of an old wizarding family, many of whom were somewhat receptive to Lord Voldemort's viewpoints. Her sister and brother-in-law, Narcissa and Lucius Malfoy, also join the Death Eaters. But whereas the average Death Eater is portrayed as power hungry and thuggish, Bellatrix is portrayed as completely evil, sadistic and insane. She spent a dozen years in Azkaban, which probably has something to do with her insanity, but since she was sentenced there for committing torture, she probably was insane to start with. Unlike her sister, who still maintained some normal attitudes, Bellatrix seemed to be totally devoted to Voldemort, and only seems to enjoy torture and murder. She also has the habit of talking to people in an exaggerated "baby" voice while in the habit of torturing, tormenting and murdering them.
The books having somewhat of a sense of justice, she falls to ruin in the final book.