Nagini

Nagini is a green snake who is Voldemort's pet, and the quaternary antagonist of the Harry Potter series. She first appeared on screen in the film adaption of Harry Potter and the Goblet of Fire. She appears at the beginning of the movie and tells Voldemort, Barty Crouch Jr. and Wormtail that Frank Bryce is outside of the door. This lead to Frank's death.

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 Goderic's Hallow. Later, Harry indeed arrives at Goderic'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. Bathilda then changes back to Nagini and tries to attack Harry, who's defending himself with a chair. Harry yelled for help and Hermione came to the rescue. Later in the same book, Nagini is possibly 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 is portrayed by 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.
 * There is said that, in Harry Potter and the Deathly Hallows: part 2, Nagini will chase 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 stab Nagini with a fang from the Basilisk. They both fail.
 * If you look 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 was unlikely due to the released snake was friendly and talk to Harry in English.
 * 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' ability to leap into other bodies) and both of their deaths cause the downfall of their masters.