Bellatrix Lestrange was born Bellatrix Black, first cousin to Sirius Black, and sister to Narcissa Black and Andromeda Black.
Bellatrix became a Death Eater and married Rodolphus Lestrange, brother of Rabastan Lestrange. By that time, she had become a sadist. One year after Lord Voldemort's defeat, the three of them (and presumably Barty Crouch Jr. as well) tried to discover information about the location of the discorporate spirit of Lord Voldemort, by using the Cruciatus Curse on Frank and Alice Longbottom. Their victims went insane from the torture, and for that crime the four of them were convicted to life imprisonement in Azkaban.
Fourteen years later, the three of them were amongst the imprisoned Death Eaters who broke out and rejoined Lord Voldemort. The Lestranges are considered some of Voldemort's most devoted followers.
Bellatrix Lestrange was in a group of Death Eaters who tried to steal a prophecy belonging to the Department of Mysteries in the Ministry of Magic from Harry Potter. She also murdered Sirius Black in a duel at the Ministry. She escaped from the Ministry with Lord Voldemort, and is presumably now with him at the end of Harry Potter and the Order of the Phoenix.
To date, she is the only femaleDeath Eater mentioned in the series specifically by name, though Narcissa Malfoy may also be one.
One of the principal themes in the novels is this juxtaposition of the magical and the mundane; the characters in the stories live utterly normal lives with utterly normal problems, despite their magical surroundings.
Sirius Black: Best friend of James Potter and former rebellious youth who fled his pure-blood supremacist parents' home in his youth.
The first four films were scripted by Steve Kloves with the direct assistance of Rowling, though she allowed Kloves what he described as "tremendous elbow room".