Severus Snape

In the first Potions class, Snape gives a speech about bottling fame and putting a stopper in death, then grills Harry with questions he can't answer, like the difference between monkshood and wolfsbane. Hermione keeps her hand up the whole time, and he ignores her.

The trio spends the whole year suspecting him. On Halloween, Snape shows up limping, his leg wounded by Fluffy. In Harry's first Quidditch match, his broom goes out of control while Snape mutters without blinking in the stands, and Hermione sets his robes on fire. In the end, Harry learns the culprit was Quirrell: Snape was muttering a countercurse to protect him.

When Harry and Ron arrive at Hogwarts in Mr. Weasley's flying car, Snape would love to see them expelled, but they get off with detention. At Gilderoy Lockhart's dueling club, Snape plays the assistant and knocks Lockhart flat with an Expelliarmus on the first try. Then he pairs Draco against Harry and whispers a tip to the Slytherin, who conjures a snake. Harry speaks to it in its own language, and Snape watches him closely.

Snape is Neville's boggart, which Neville turns, in Lupin's class, into a Snape wearing his grandmother's clothes. Snape can't stand Lupin. On a day his colleague is sick, Snape takes over Defense Against the Dark Arts and assigns everyone reading on werewolves. When he catches Harry with the Marauder's Map, the parchment insults him and Lupin steps in.

In the Shrieking Shack, Snape arrives ready to hand Sirius Black over to the Dementors. It's clear that he, Sirius and Lupin know each other from school, and that there was no love lost. Harry knocks him out with an Expelliarmus. At the end of the year, Lupin resigns after his secret gets out.

Ingredients go missing from Snape's private stores, and he accuses Harry and threatens to use Veritaserum, the truth potion, to make him confess. In Dumbledore's Pensieve, Harry watches the trial of Igor Karkaroff, a former Death Eater who gave up names to get out of Azkaban. Karkaroff names Snape as a Death Eater, and Dumbledore vouches that Snape switched sides and spied for them before Voldemort's fall.

When the fake Mad-Eye Moody is unmasked, it's Snape who brings the truth potion that makes Barty Crouch Jr. confess.

On Dumbledore's orders, he gives Harry private Occlumency lessons to shut his mind against Voldemort. The lessons are a disaster. In one of them, Harry turns the spell back on his teacher and sees memories of a teenage Snape being bullied by James Potter, Harry's father, in front of other students. Snape throws him out and ends the lessons.

Trapped in Umbridge's office, Harry tells Snape, in code, that "he's got Padfoot." Snape pretends not to understand in front of her.

Narcissa Malfoy and her sister, Bellatrix Lestrange, visit Snape at his house on Spinner's End. Narcissa asks him to protect Draco, who has been given a mission by Voldemort. With Bellatrix as witness, Snape makes the Unbreakable Vow: if Draco fails, he'll finish the job. At Hogwarts, Slughorn takes over Potions. When Harry uses Sectumsempra on Draco in the bathroom, Snape arrives and heals the boy.

At the top of the Astronomy Tower, Draco disarms Dumbledore but can't bring himself to kill him. Snape arrives, Dumbledore says "Severus, please," and Snape kills him with the Killing Curse. As he flees, Harry attacks him with spells from the Potions book, and Snape reveals: he is the Half-Blood Prince.

Snape tells Voldemort the date Harry will leave his aunt and uncle's house and becomes headmaster of Hogwarts. When Harry comes back to the school, McGonagall duels him in the Great Hall and Snape escapes through a window. Voldemort summons him to the boathouse and, convinced the Elder Wand will only obey him once Snape is dead, orders Nagini to attack. Harry arrives in time to hear his last words: Snape gives him his tears, full of memories, and asks Harry to look at him, because he has his mother's eyes.

In the Pensieve, the truth comes out. Snape had loved Lily since childhood. When Voldemort killed her, it was Snape who wept holding her body in Godric's Hollow, and from then on he protected her son at Dumbledore's request, pretending to serve Voldemort. Dumbledore's death had been arranged between the two of them. In one of the memories, Snape casts a silver doe, the same shape as the Patronus that led Harry to the sword of Gryffindor. Dumbledore asks if, after all this time, he still loves her, and Snape answers: "Always." Nineteen years later, Harry tells his son Albus Severus that he was named after one of the bravest men he ever knew.

In the books, Snape has greasy black hair, a hooked nose, sallow skin and crooked teeth, a long way from Alan Rickman. His dislike of Harry is even heavier on the page, with more classroom and detention scenes. J.K. Rowling told Rickman a detail of the character's past years before the last book came out, and he kept the secret.