Nagini is a green snake who is Voldemort's pet, and is the tertiary antagonist of the Harry Potter series, mainly in Deathly Hallows. 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 leads 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 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.