Severus Snape

Severus Snape first appears in Harry Potter and the Philosopher's Stone, shortly after Harry Potter arrives at Hogwarts. He is the school's Potions Master, though he is widely rumoured to covet the Defence Against the Dark Arts post. Snape himself confirms the rumour in Harry Potter and the Order of the Phoenix. Snape is a sinister and malicious teacher, who makes frequent snide and disparaging remarks at Harry's expense. He quickly becomes one of the main antagonists of the book, as Harry suspects him of plotting to steal the philosopher's stone, and of attempting to kill him. Only the climax of the book reveals that Professor Quirrell, in league with Lord Voldemort, is the real enemy while Snape, suspicious of Quirrell, was looking out for Harry throughout the book. In the final chapter, Dumbledore suggests that because Harry's father James had saved Snape's life when they were both students, even though the two detested each other, Snape felt responsible for Harry in return. As the final book reveals, this is not the full story. In any case, even after Quirrell's true role is revealed, Harry retains feelings of suspicion and resentment towards Snape, and their relationship remains tense. Snape's behaviour and attitude towards Harry also remain unchanged.

Snape has a minor role in Harry Potter and the Chamber of Secrets, where he helps Gilderoy Lockhart oversee Hogwarts' short-lived Duelling Club but has little interaction with the main plot. It is while attending the Duelling Club that Harry learns the Expelliarmus spell indirectly from Snape, a spell that plays a significant role in later books.

In Harry Potter and the Prisoner of Azkaban, Snape demonstrates his expertise with potions by brewing the complex Wolfsbane potion for the new Defence Against the Dark Arts professor, Remus Lupin. Throughout the third book, Snape suspects that Lupin may be helping Sirius Black enter Hogwarts castle; Sirius had been convicted (falsely, as it is later revealed) for murdering innocent bystanders and betraying the Potter family's hiding place to Voldemort. This suspicion stems from Lupin's friendship with Sirius and Harry's father James while they were all at Hogwarts as students. Near the climax of the book, Snape attempts to apprehend Black, but Black escapes with Harry's aid. Snape informs Dumbledore of this and when Harry and Lupin are not punished, Snape retaliates by revealing to the entire school that Lupin is a werewolf, forcing the latter to resign his post.
Prisoner of Azkaban reveals more details about the connection between Snape and James Potter. While in school together, Sirius once tricked Snape into entering the Shrieking Shack while Lupin was there, transformed into a werewolf. James realised the danger and stopped Snape, saving his life; this is the incident Dumbledore referred to at the end of the first book. Snape, however, believes James's actions were self-serving, to avoid being expelled.
Snape's role in the fourth novel, Harry Potter and the Goblet of Fire is not substantially different from that of the previous two books. Although he is apoplectic when Harry is unexpectedly entered into the Triwizard Tournament, this only makes him even more vindictive than usual. Later Harry accidentally falls into Dumbledore's Pensieve and views memories of several Death Eater trials from years before. At one point, Snape is named as a Death Eater by Igor Karkaroff, but Dumbledore comes to Snape's defence, claiming that although Snape had indeed been a Death Eater, he changed sides before Voldemort's downfall and turned spy against him. Later, Dumbledore assures Harry that Snape's reformation is genuine, though he refuses to tell Harry how he knows this, saying the information "is a matter between Professor Snape and myself".

At the end of the book, Dumbledore attempts to convince a disbelieving Minister for Magic, Cornelius Fudge, that Voldemort has returned. As proof, Snape willingly shows Fudge the restored Dark Mark on his arm, and is subsequently sent on a secret mission by Dumbledore. This mission, as implied in Harry Potter and the Order of the Phoenix and revealed in Harry Potter and the Half-Blood Prince, was to rejoin the Death Eaters and spy on Voldemort as a triple agent, pretending to spy on Dumbledore on behalf of Voldemort.
In the fifth novel, Harry Potter and the Order of the Phoenix, Snape returns to a more prominent role. With Voldemort having returned to a fully corporeal body, Snape continues working as a triple agent for Dumbledore. He is seen prior to the start of school at Number 12, Grimmauld Place giving reports to the Order of the Phoenix. He has a very strained relationship with Sirius, who owns Grimmauld Place and must remain there in hiding. The two trade frequent snide remarks and at one point almost begin a duel. Snape taunts Sirius about the latter not being able to take an active role in the Order's missions due to his fugitive status. Harry later feels that this contributed to Sirius's willingness to take unsafe risks. Back at school, Snape's allegiance to the Order has no effect on his dislike for Harry.

Later in the book, Dumbledore has Snape teach Harry Occlumency, the protection of the mind from outside intrusion or influence. The sessions are made difficult by their mutual hostility and end prematurely when Harry uses Dumbledore's Pensieve to view one of Snape's childhood memories without the latter's permission. He sees the memory of Snape being bullied by James and Sirius, and of calling Lily Evans a Mudblood. Only in the final book is it revealed that, prior to this confrontation, Snape and Lily had been close friends.

Towards the end of the novel, Dolores Umbridge captures Harry and questions him on the whereabouts of Dumbledore. She sends for Snape to provide a truth serum to force Harry to reveal any information he may be hiding. Snape claims that his supplies of Veritaserum were exhausted earlier, when she attempted to use the drug surreptitiously to force information from Harry. Snape withholds further assistance. It is later revealed that Snape had in fact supplied Umbridge with fake Veritaserum on the prior attempt. Snape then carries Harry's cryptic warning about Sirius's capture to the other Order members, allowing them to come to the rescue in the Department of Mysteries. Harry still holds Snape partly responsible for Sirius's death, believing Snape's goading spurred Sirius into joining the battle.
In the second chapter of Harry Potter and the Half-Blood Prince, Bellatrix Lestrange and Narcissa Malfoy visit Snape at his home in Spinner's End. Narcissa's son Draco has been given a difficult task by Voldemort, and Narcissa swears Snape to an Unbreakable Vow that he will protect Draco, help him complete Voldemort's task, and finish the task himself if Draco fails. When questioned by Bellatrix about his loyalties, Snape claims to have been working for Voldemort (rather than for Dumbledore) ever since Voldemort's return, and explains his actions in the previous books in that light. In addition, he points out that Dumbledore's protection has kept him out of Azkaban and free to operate on Voldemort's behalf.

At the start-of-term feast at Hogwarts, Dumbledore announces he has finally appointed Snape as Professor of Defence Against the Dark Arts. Horace Slughorn, a retired Hogwarts teacher, replaces Snape as Potions Master. Slughorn lends Harry an old Potions textbook, in which Harry finds marginalia including a variety of hexes and jinxes seemingly invented by an unknown student, and substantial improvements to the book's standard potion-making instructions. The book is inscribed This Book is the Property of the Half-Blood Prince. The notes greatly bolster Harry's performance in Potions and he impresses Slughorn. Snape, who maintains that he "never had the impression that [he] had been able to teach Potter anything at all", is suspicious of Harry's newfound Potions success.

Later, in a fight with Draco, Harry casts one of the Prince's spells (Sectumsempra) marked "For Enemies" and is horrified when it causes devastating wounds to Draco's face and chest. Snape rushes to the scene and heals Draco's wounds, and then interrogates Harry about the source of the spell, using Legilimency to extract the source of Harry's knowledge (the Potions textbook) from Harry's mind. When Snape insists that Harry show him his Potions textbook, Harry hides the Prince's book and gives Ron Weasley's book to Snape instead. As punishment for Harry's attack on Malfoy and knowing Harry is lying about the textbook, Snape puts Harry in detention during the final Quidditch match of the year.

Returning to Hogwarts after a search for one of Voldemort's Horcruxes, Harry and Dumbledore alight on the school's astronomy tower. Gravely weakened by Voldemort's protective potion, Dumbledore asks Harry to fetch Snape. Before Harry can leave, Draco suddenly arrives intending to carry out Voldemort's ordered assassination of Dumbledore, but cannot bring himself to commit the murder. The Death Eaters arrive and Snape interrupts them, killing the headmaster himself. An enraged Harry (who had been paralysed by Dumbledore and witnessed the killing while under his invisibility cloak) chases Snape, Draco, and the Death Eaters as they flee the castle. Snape easily blocks Harry's attempts to attack him with magic and even jeeringly points out Harry's mistakes, but refuses to strike back. During the confrontation, Snape reveals himself to be the "Half-Blood Prince" (being the son of Muggle Tobias Snape and pure-blood Eileen Prince). Harry is unable to stop Snape before the latter passes through the school gates and Disapparates. The full story of the relationship between Dumbledore and Snape and the real reason for the killing are not revealed until the next and last book. Rowling mentioned in an interview that at this point in the series, the Harry-Snape relationship has become "as personal, if not more so, than Harry-Voldemort."
n Harry Potter and the Deathly Hallows, Snape is named Headmaster of Hogwarts, while Death Eaters Alecto and Amycus Carrow are appointed to the Hogwarts staff. The novel later reveals that Snape uses his position as Headmaster to protect the students and to contain the Carrows. In the course of the book, Harry and Ron are led to find the Sword of Godric Gryffindor by a Patronus taking the form of a doe.[38] Harry later learns that this was Snape's Patronus, taking the same shape as Harry's mother Lily's Patronus, and that Snape had been tasked by Dumbledore with ensuring that Harry gained possession of the sword.

Towards the end of the school year, Professors McGonagall, Flitwick, and Sprout force Snape to flee the school. Voldemort summons Snape to the Shrieking Shack. Erroneously believing Snape is the master of the Elder Wand, Voldemort betrays Snape and has his pet snake Nagini bite him through the neck, mortally wounding him, believing that Snape's death will make him the master of the Wand. Snape, dying from his wounds, releases a cloud of memories and tells Harry, who has watched the entire scene from a hidden spot, to take them. From these memories, Harry sees Snape's childhood and learns his true loyalties. In this vision, Harry learns that Snape befriended Lily as a child when they lived near each other. Upon their arrival at Hogwarts, the Sorting Hat placed Snape and Lily into Slytherin and Gryffindor Houses, respectively. They remained friends for the next few years until they were driven apart by Snape's interest in the Dark Arts; the friendship finally ended following the bullying episode that Harry had briefly seen in the fifth book. Despite this separation and Snape's animosity toward Lily's eventual husband James, Snape remained in love with Lily.

Harry learns that Snape had revealed the prophecy made by Sybill Trelawney (not knowing, at first, that it was referring to Lily and her family) to Voldemort, prompting the Dark Lord to attack the Potters in an attempt to prevent its fulfilment. Though he asked Voldemort to spare Lily, Snape, still fearing for her safety, went to Dumbledore and begged him to protect the Potters. Dumbledore agreed and ensured that they were placed under the Fidelius Charm. In return, Snape became a triple agent for the Order of the Phoenix against Voldemort, using his powers of Occlumency to hide his betrayal from his master. Even with his efforts to protect her, Snape felt responsible for Lily's death when the Fidelius charm was broken. Snape demanded of Dumbledore, however, that his love for Lily — his reason for switching sides — be kept a secret. Dumbledore agreed and kept the secret throughout the series.

Snape's memories then reveal that Dumbledore had been afflicted by a powerful curse cast on the Gaunt ring, one of Voldemort's Horcruxes, prior to the start of Harry's sixth year at Hogwarts. Although Snape's knowledge of the Dark Arts enabled him to slow the spread of the curse, the curse would have ultimately killed Dumbledore within a year. Dumbledore, aware that Voldemort had ordered Draco to kill him, asked Snape to kill him instead as a way of sparing the boy's soul and of preventing his otherwise slow, painful death. Although Snape was reluctant, even asking about the impact of such an action on his own soul, Dumbledore implied that this kind of mercy killing, or killing "by request", would not damage a human's soul in the same way murder would. Snape agreed to do as the Headmaster requested. Snape's memories also provide Harry with the information he needs to ensure Voldemort's final defeat, in the form of conversations Snape had with Dumbledore.

Rowling noted in an online interview that because Snape abandoned his post before dying or officially retiring, a portrait of him does not immediately appear in the Headmaster's office following his death. She adds, however, that she would like to think Harry made Snape's true loyalty and heroism known in the Wizarding world, and that he lobbied to ensure that a portrait be installed in the office. In a separate interview, Rowling discussed Snape's back story, saying she had planned it ever since she wrote the first book because the whole series is built around it and she considers him one of the most important characters of the seventh book.