Muggles' Guide to Harry Potter/Characters/Nagini

Overview
Nagini is a snake, a large one, and Lord Voldemort's pet.

Goblet of Fire
We first see Nagini in Riddle Manor, to which Lord Voldemort has returned while he prepares for his full revival. Nagini, out hunting, returns to Voldemort, passing Frank Bryce in the hall, and reports on Frank's presence to Voldemort, whereupon Voldemort has Wormtail bring Frank into the room. Voldemort then kills Frank. The conversation between Voldemort and Wormtail leads us to believe that Wormtail is milking Nagini for something, most likely venom, which is keeping Voldemort alive.

When Harry is bound to the gravestone in the climactic battle at the end of the book, Nagini circles around him; Voldemort, now returned, promises Nagini that she will be allowed to eat Harry after the evening's proceedings are done.

Order of the Phoenix
While it is never stated explicitly, it is almost certain that Nagini is the snake that attacks Arthur Weasley just before Christmas Day.

Half-Blood Prince
In discussing the probable artifacts that have been turned into Horcruxes, Professor Dumbledore suggests to Harry that Voldemort's pet snake is acting rather suspiciously un-snake-like, and quite possibly is carrying a Horcrux. Harry asks if living creatures can be Horcruxes, and Dumbledore says that they can, although it is risky, of course, as living things can be killed.

Deathly Hallows
Nagini is present at the council meeting with Voldemort and his Death Eaters. Voldemort, testing Lucius Malfoy's wand, kills Charity Burbage, then tells Nagini that it is dinner time. Nagini then consumes Charity's dead body.

When Harry and Hermione are visiting Godric's Hollow, they meet with Bathilda Bagshot. Bathilda invites them to her house, then gets Harry to follow her to a room upstairs. It turns out that Bathilda is, in fact, dead, and is being animated by Nagini who is inside her. Nagini notifies Voldemort that she has trapped Harry, then emerges from the remains of Bathilda and attacks. While she bites Harry, her instructions are to hold him, so she does not poison him. With Hermione's help, Harry manages to escape from Nagini.

When Voldemort learns that his Horcruxes are being systematically stolen and destroyed, he tells Nagini that she will have to stay close by him for a while. Apparently, she is with him as he checks the places where his Horcruxes had been hidden.

In the Shrieking Shack, when we see Voldemort, Nagini is by his side, twining restlessly within an apparent protective spell. Severus Snape is summoned to Voldemort's presence, and Voldemort says that the Elder Wand, which he has taken from Dumbledore's tomb, is not working as well as he had expected. Voldemort has come to the conclusion that, because Snape had killed Dumbledore, the wand owed its allegiance to Snape, and would do so until someone killed Snape. Whereupon, he throws Nagini, protective spell and all, at Snape. Snape is bitten viciously, and Voldemort and Nagini leave, allowing Harry time to receive memories from the fallen Snape. In Snape's memories, we see Dumbledore telling Snape that at some point, Voldemort will suddenly start being very protective of Nagini, keeping her always by his side. Then, and only then, will it be time to tell Harry all that Dumbledore has told Snape.

As Harry heads out of the school for his meeting with Voldemort, he runs into Neville Longbottom. Thinking of Dumbledore and his backup plans, Harry tells Neville that it is vital that, should anything happen to Harry, Nagini must be killed. This is necessary before Voldemort could be defeated forever.

When Harry confronts Voldemort in the Forbidden Forest, he sees Nagini, still twining restlessly within the protective spells Voldemort has cast about her. He thinks about the possibility of killing her, but determines that it is too risky; with the protective spells, and the large number of Death Eaters present, he is unlikely to get an effective spell out in time.

As Voldemort is crowing before the school about his final victory over Harry, Harry notices that Nagini is no longer encased within the protective spell. Neville produces the Sword of Gryffindor out of the Sorting Hat and with it, decapitates Nagini, killing her.

Strengths
Being a snake, Nagini has all the strengths of snakes: venom, physical strength, ability to travel through small apertures, maneuverability. Additionally, sharing some of Voldemort's soul, she also retains some of his intelligence, and due to her large size and possibly magically-enhanced venom, her bite is powerful enough to kill a full-grown man in seconds, or cause one to bleed to death in minutes.

Weaknesses
Being a snake, her intelligence and ability to plan are limited, despite her sharing of Voldemort's soul and part of his intellect.

Relationships with Other Characters
Nagini is Voldemort's pet, and contains one of his soul fragments. As such, she is extremely close to him. Towards most others, she is very much aloof, though she is likely to attack those who are in her, or Voldemort's, way.