Bellatrix Lestrange (née Black) was the first female Death Eater introduced in the books, and remained the only woman explicitly identified as such until Harry Potter and the Half-Blood Prince. Bellatrix was born to Cygnus and Druella (Rosier) Black 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 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.[8] Indeed, she seems very indifferent about her husband, and never actually refers to him in any of the books. It is suggested in the novels that, as a student, Bellatrix associated with a group of Slytherins - including Rodolphus, Snape, and Avery - who nearly all became Death Eaters. It is assumed Bellatrix was at least initially drawn to Voldemort because they both believe in an ideology that favors pure-blood wizards and witches over other members of the community.[9] Bellatrix, her husband and her brother-in-law were active Death Eaters in Voldemort's rise to power, and evaded capture and suspicion until after the Dark Lord's downfall at the beginning of the series.

In Harry Potter and the Goblet of Fire, Rowling used Albus Dumbledore's Pensieve as a plot device to reveal that Bellatrix, rather than deserting her leader like many other Death Eaters, was part of the group of dark wizards - along with Rodolphus, Rabastan, and Barty Crouch Jr - that tortured well-known Aurors Frank and Alice Longbottom in an attempt to gain information about Voldemort's location.[10] For using the Unforgivable Cruciatus Curse to torture the Longbottoms until they went insane, Bellatrix and her three associates were sentenced to life imprisonment in Azkaban. Later in that book, during his rebirthing ritual, Voldemort stated that the Lestranges were amongst the most faithful members of his inner circle.[11]

Fourteen years after Voldemort's fall, Bellatrix was one of the many Death Eaters who escaped Azkaban and rejoined him. After escaping from prison, she was 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 attempted to steal Sybill Trelawney's prophecy pertaining to Voldemort's downfall. Bellatrix shows up with Malfoy at the prophecy hall and battles the group of teenagers. Rowling let Bellatrix prove her magical prowess during the mission when she overpowered her niece Tonks and Kingsley Shacklebolt in one-on-one duels, killed her cousin Sirius by blasting him through The Veil in the Death Chamber, and deflected one of Dumbledore's spells as she made her escape. Harry attempted to use the Cruciatus Curse on her in revenge for killing Sirius, but the curse was ineffective due to the lack of real cruelty behind it. Before she could do any more, Bellatrix was joined by her master, who ignored her warning that Dumbledore was in the building. Bellatrix was subdued by Dumbledore in the Ministry of Magic's Atrium while he dueled Voldemort. Voldemort interceded on Bellatrix's behalf, grabbing her and taking her with him as he Disapparated, though not before being glimpsed by Ministry officials.

In the beginning of Harry Potter and the Half-Blood Prince, Bellatrix attempted to keep Narcissa from confiding Draco's secret mission to Severus Snape at Spinner's End. Rowling used the conversation between Snape and Bellatrix to imply that Voldemort was still furious at Bellatrix's failure in the past book[12]. That conversation also suggested that Bellatrix mistrusted Snape not only because of his low birth, but also for many valid questions about his loyalty to the Dark Lord. Snape surprised Bellatrix by replying well-thought arguments to her and by asking her to bind him and Narcissa with an Unbreakable Vow to assist Draco in his mission to kill Dumbledore.

The first chapter of the final Harry Potter book implies that Voldemort is still furious at Bellatrix, as evidenced when he makes fun of the fact that her niece Tonks married werewolf Remus Lupin.[13] However, Voldemort gives Bellatrix a chance to "clean" 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.[14] In this book, Rowling revealed that Bellatrix was the guardian of Helga Hufflepuff's cup (though she is unaware of its true nature), which Voldemort had 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 the elf 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, 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.[15] When the battle resumes inside the Great Hall after Harry's supposed death, Bellatrix simultaneously duels Hermione, Ginny Weasley and Luna Lovegood, all of whom are no match for Bellatrix, who nearly hits Ginny with a Killing Curse. An enraged Molly Weasley engages Bellatrix in a duel to kill and fires a curse that finally defeats Bellatrix.

Bellatrix is described as having been very attractive in her youth, but her great good looks were hollowed during her years in Azkaban,and she is left a remnant of her former beauty. As a result, 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. However, she is a witch of prodigious ability, as demonstrated by all her victories in duels against other characters, and also as noted by Harry in the final book[16], and seems to actually be far more intelligent than many of her fellow Death Eaters. An example is her suspicion about Snape, which seems to be founded in paranoia and jealousy at the time, but is later found to be justified.[12]