Nagini

"I think he is perhaps as fond of her as he can be of anything. He certainly likes to keep her close and has an unusual amount of control over her, even for a Parselmouth."

- Albus Dumbledore on Voldemort's bond with Nagini.

Nagini is a long green female snake who is Lord Voldemort's pet, and a major antagonist in the Harry Potter series, mainly in Deathly Hallows.

Background
Not much was known about Nagini's early life or when she was acquired by Voldemort. Originally, Nagini was a human Maledictus woman capable of transforming herself into a snake. In 1927, she was a featured attraction in a wizard circus owned by Skender, where she became friends with Credence Barebone. Finally, her blood curse turned her into a snake. It was stated that in the woods of Albania, the phantom-like form that was the remnants of Voldemort had befriended many animals, yet only snakes could bear having him inhabit them, thus leading to the notion that Voldemort and Nagini's relationship had originally commenced in Albania.

Personality
Nagini was completely loyal to Lord Voldemort. While she remained aloof, if not malevolent, to most others, she obeyed Voldemort completely, and seemed to enjoy being close to him, as she often slithered up to his shoulders to receive his affection.

She was also shown to be very intelligent and was able to understand the concept of strategies and plans, as seen when she set up a trap for Harry (which may have been due to her possessing a portion of Voldemort's soul) by disguising as Bathilda. She was rather cunning and deceptive, and seemed to have understood how humans behaved.

History
Since acquiring her, Voldemort used Nagini to help him regain strength and a basic physical form after living in a weak and spectral state, following his ill-attempted murder on an infant Harry Potter. He had the servant who returned to him, Peter Pettigrew, milk her venom, and with it, create a potion that sustained him with a temporary body until he could regain his true form. Nagini further kept Voldemort alive after he turned her into a Horcrux, with the murder of Bertha Jorkins, in 1994; he was as attached to her as he could of any living creatures, and her underlining of his heritage to the legendary Salazar Slytherin only furthered her mystique to him. His habit of keeping her unusually close to him was what led Albus Dumbledore to suspect that she had become a Horcrux for quite sometime.

In the fourth book, Nagini, along with Voldemort and Pettigrew, sought shelter in the abandoned Riddle House until they could instigate the plan to capture Harry Potter via botching Triwizard Tournament. Nagini alerted Voldemort to the presence of an Frank Bryce, an old Muggle gardener who had worked for the late Riddle family, who had overheard a discussion between Pettigrew and Voldemort. Soon after, Nagini slithered past Bryce and alerted Voldemort, and the Dark Lord murdered the Muggle man.

Upon Voldemort's rebirth via the capture of Harry, Nagini was slithering around the Little Hangleton graveyard that the ceremony took place, and was promised that she could feed on the boy's corpse after her master had killed him. She was deprived of this promise when Harry escaped however.

In the fifth book, Nagini was presumably sent by the Dark Lord to retrieve a Prophecy in the Ministry of Magic. After she had infiltrated the area, Nagini met up with Order of the Phoenix member, Arthur Weasley, who was sent to guard the very same prophecy. Nagini, unable to resist, and who had attracted Arthur's attention already, attacked him. The attack was witnessed by Harry in one of his dreams, and felt that he (Harry) was the snake. Critically injured, Arthur was taken to St Mungo's. He survived the attack, but was scarred as result. The reason that Albus Dumbledore believed Harry saw the vision as if he was the snake was due to Harry’s special connection to Voldemort, with Harry’s witnessing the attack by virtue of the fact that Voldemort’s mind "happened to be" in Nagini at the time. This was the first indication of Nagini and Voldemort’s deeper connection, having the ability to share thoughts and connect with Harry (which in final book confirmed that Harry was accidently became one of Voldemort's horcruxes through ill-attempted murder on him when he still baby).

In the seventh and final book, Voldemort uses Nagini to possess Bathilda Bagshot, who was killed earlier. He knew that Harry Potter was coming to Godric's Hallow. Later, Harry indeed arrives at Godric's Hallow with Hermione. Nagini, In Bathilda's body, brings them to Bathilda's house and takes Harry upstairs into a dark room.

There she contacts Voldemort and Voldemort tells her to keep Harry with her. Nagini then bursts from Bathilda’s corpse and tries to attack Harry, who ends up receiving a puncture wound to the arm. Later in the same book, Nagini is killed by Neville Longbottom during the battle of Hogwarts. Neville did this since Harry ordered him to kill Nagini because she was the last Horcrux remaining.

Nagini is also responsible for the death of Severus Snape, after Voldemort ordered her to kill him.

Trivia

 * Bathilda Bagshot was portrayed by the late Hazel Douglas in Harry Potter and the Deathly Hallows: part 1. But since Hazel only portrays Bathilda in the scenes where she is possessed by Nagini, it could be said that she portrays Nagini as well.
 * In the movie version of Harry Potter and the Deathly Hallows: part 1, Harry hits Nagini on the head with a brick. He also uses a chair to defend himself against Nagini's attack. This never happened in the book.
 * In Harry Potter and the Deathly Hallows: Part 2 Nagini chases Ron and Hermione around the castle, this also never happened in the book.
 * Nagini's species is never revealed in the books - she is stated to be venomous, but much larger than any venomous snake in real life (with the possible exception of the King Cobra, but the books never mention any cobra-like features, such as a hood). It is most likely that she is a magical species unknown to muggles, perhaps even created by hybridizing several species of snake. In the movies she is portrayed by a CG snake, but her design is heavily inspired on pythons - Burmese python in Harry Potter and the Goblet of Fire, and Reticulated python in Harry Potter and the Deathly Hallows (both parts).
 * In Harry Potter and the Deathly Hallows: Part 2, both Ron and Hermione try to kill and stab Nagini with a fang from the Basilisk. They both fail.
 * If anyone looks closely to the scene where Harry walks away from the mirror in Bathilda's bedroom in Harry Potter and the Deathly Hallows: Part 1, you can see the final part of the transformation in the mirror image.
 * In the movie version, Harry doesn't yell for Hermione when Nagini appears in her normal form. Hermione spots blood on the ceiling and realizes something is not right herself, after which she decides to go take a look upstairs.
 * There is a Simpsons pastiche of Nagini called Slithers.
 * There is fan-speculation that the snake Harry released at the zoo in The Sorcerer's Stone was in fact Nagini. However, the speculation is false due to the released snake being a male Burmese and Nagini was female and likely came into Voldemort's companionship in Albania.
 * The Red Lyrium Dragon bears a similar role to Nagini as both are female reptiles who also act as both pets and sources of their masters immortality (Nagini as one of Voldemort's Horcruxes and Red Lyrium Dragon as the source of Corypheus's ability to leap into other bodies) and both of their deaths cause the downfall of their masters.