Bellatrix Lestrange

Bellatrix Lestrange (née Black) is the first female Death Eater introduced in the books, and one of the most faithful members of Voldemort's inner circle. She is described as having been very attractive in her youth, but her patrician good looks are hollowed during her years in Azkaban, and she is left a remnant of her former beauty. Bellatrix is often portrayed as looking slightly mad and, more than once, it is implied that she is paranoid, mentally unstable and fanatically devoted to Voldemort. Despite her apparent mental instability, she is a witch of prodigious ability, as demonstrated by all her victories in duels against other characters, and as noted by Harry in the final book.
Bellatrix was born to Cygnus Black and Druella Rosier in 1951, and has two younger sisters, Andromeda and Narcissa. She is also a first cousin to Sirius and Regulus Black, and an aunt to Draco Malfoy and Nymphadora Tonks. Bellatrix married Rodolphus Lestrange a few years after leaving Hogwarts "because it was expected of her" to marry a pure-blood. However, Rowling stated in an interview that Bellatrix truly loved Voldemort. In Harry Potter and the Goblet of Fire, Rowling uses Dumbledore's Pensieve as a plot device to reveal that Bellatrix, rather than deserting Voldemort after his downfall like many other Death Eaters, is part of the group of dark wizards – along with Barty Crouch Jr, Rodolphus and Rabastan Lestrange – that torture well-known Aurors Frank and Alice Longbottom in an attempt to gain information about Voldemort's location. For using the Unforgivable Cruciatus Curse to torture the Longbottoms until they go insane, Bellatrix and her three associates are sentenced to life imprisonment in Azkaban.
Fourteen years after Voldemort's fall, Bellatrix is one of the many Death Eaters who escaped Azkaban and rejoined him. She is present at the battle of the Department of Mysteries in the climax of Harry Potter and the Order of the Phoenix, in which a group of Death Eaters attempt to steal Sybill Trelawney's prophecy pertaining to Voldemort's downfall. Rowling let Bellatrix prove her magical powers during the mission when she overpowers her niece Tonks and Kingsley Shacklebolt in one-on-one duels, kills her cousin Sirius by blasting him through the veil in the Death Chamber, and deflects one of Dumbledore's spells as she makes her escape. Harry attempts to use the Cruciatus Curse on her in revenge for killing Sirius, but the curse is ineffective due to the lack of real cruelty behind it. Before she can do any more, Bellatrix is subdued by Dumbledore in the Ministry of Magic's Atrium while he duels Voldemort. Voldemort intercedes on Bellatrix's behalf, grabbing her and taking her with him as he Disapparates, though not before being glimpsed by Ministry officials.
In the beginning of Harry Potter and the Half-Blood Prince, Bellatrix attempts to keep Narcissa from confiding Draco's secret mission to Snape at Spinner's End. Rowling used the conversation between Snape and Bellatrix to imply that Voldemort is still furious at Bellatrix's failure in the past book. That conversation also suggests that Bellatrix mistrusts Snape not only because of his low birth, but also for many valid questions about his loyalty to the Dark Lord. Snape surprises Bellatrix by replying to her with arguments and by agreeing to create an Unbreakable Vow with Narcissa to assist Draco in his mission to kill Dumbledore.
The first chapter of Harry Potter and the Deathly Hallows implies that Voldemort is still angry at Bellatrix, as evidenced when he makes fun of the fact that her niece Tonks married werewolf Remus Lupin. However, Voldemort gives Bellatrix a chance to "trim" her family tree during the Death Eaters' attempt to capture Harry as the boy departs from the Dursleys' home, in which Bellatrix unsuccessfully tried to kill Tonks. In this book, Rowling reveals that Bellatrix is the guardian of Helga Hufflepuff's cup (though she is unaware of its true nature), which Voldemort has entrusted the Lestranges to keep in their Gringotts vault. Bellatrix and the Malfoys detain Harry, Ron, and Hermione at Malfoy Manor, and Hermione is tortured by Bellatrix when she suspects the trio has broken into her vault, but Dobby appears and saves the prisoners, though not before being hit by a knife thrown by Bellatrix as they Disapparate to safety. Later in the book, Harry, Ron and Hermione use a stray hair of Bellatrix's to disguise Hermione as Bellatrix using the Polyjuice Potion, to get into the vault. Though Voldemort apparently punishes Bellatrix and the Malfoys severely for interrupting his Elder Wand side quest only to have Potter escape and steal the cup, she nevertheless fights for her master in the Battle of Hogwarts towards the end of the novel. Rowling stated in an interview that, during the battle, it was Bellatrix who killed Tonks. When the battle resumes inside the Great Hall after Harry's supposed death, Bellatrix simultaneously duels Hermione, Ginny and Luna, none of whom is a match for Bellatrix, who nearly kills Ginny with a Killing Curse. An enraged Molly Weasley engages Bellatrix in a duel and fires a curse that finally kills Bellatrix.