The first Harry Potter movie introduces Severus Snape at the Sorting Ceremony, seated at the High Table talking to the Defense Against the Dark Arts teacher, Quirinus Quirrell. The first book in the Harry Potter series - The Sorcerer's Stone describes Snape as "...a teacher with greasy black hair, a hooked nose and sallow skin." He is part of a brief but perplexing scene that involves him, Harry, and Quirrell. Snape looks past Quirrell's turban, straight into Harry's eyes who experiences a sharp pain across the scar on his forehead. Moments later, at the start-of-term banquet, Ron's brother, Percy Weasley informs Harry that the professor talking to Quirrell is the head of Slytherin House and is named Snape.
In his first Potions lesson, Snape waltzes into the classroom announcing, "There will be no foolish wand-waving or silly incantations in this class." He sneers at Harry and proceeds to ask, "What would I get if I added powdered root of asphodel to an infusion of wormwood?" He interrogates Harry with two more questions and deducts a point from Gryffindor for the boy's "cheek." Sweeping around in his black cloak, he calls Neville Longbottom "Idiot Boy!" for adding porcupine quills before taking his cauldron off the fire, bullies Harry some more, and chastises everyone except Draco Malfoy. While Snape is neither the friendliest of teachers, nor empathetic, he's nowhere close to the monster Harry, Ron, and Hermione think of him in the first Harry Potter movie.
Snape And Lily Through The Years
Due to Snape's negative demeanor, surly expressions, and suspicious behavior, the trio assumes he's after the Sorcerer's Stone. While the truth comes out at the end of the first Harry Potter movie, and Professor Quirrell is discovered to be the villain, Snape remains an ambiguous character until the conclusion of his arc. He's also a severely misunderstood character in the Harry Potter franchise, and therefore it's pertinent to delve into his past to find the root cause of his hostility toward Harry.
In Harry Potter and the Deathly Hallows: Part 2 Harry pours Snape's memories into the Pensieve, and finally understands the man from the boy. Snape, an English half-blood wizard was born to a witch, Eileen, and a muggle, Tobias. He grew up on the Spinner's End in a dysfunctional, poor family. He'd been observing Harry's mother, a girl named Lily Evans from afar, and noticed she had traits of a witch. The duo become friends before joining Hogwarts and awaited their acceptance letters. They talked about magic, Azkaban, and whatnot while Lily's older sister, Petunia grew jealous of them. She called them "weirdos" and Hogwarts "a school for freaks" while secretly wanting to attend it and to that end, she wrote a letter to Dumbledore begging him to take her in.
Despite a few initial misunderstandings, Snape and Lily patched up their differences and continued to be friends at Hogwarts even though he was sorted into Slytherin and she into Gryffindor (as shown in Harry Potter and the Deathly Hallows: Part 2). While Snape performed increasingly well, he excelled at Defense Against the Dark Arts and Potions. He also nicknamed himself the "Half-Blood Prince." He wasn't particularly fond of a student (and a bully) named James Potter who disparagingly called him "Snivellus" and picked on him since their first train ride to Hogwarts. Lily, meanwhile, disliked Snape's friends, Mulciber, and Avery because they practiced Dark Magic. With time, Snape grew increasingly suspicious of James and his mates as they sneaked out at night. He was certain that something was amiss about Remus Lupin who unbeknownst to most was a werewolf. Lupin's friends, James, Sirius, and Peter had secretly become Animagi by their fifth Hogwarts year to keep him in check during his monthly transformations.
Snape Joins The Death Eaters And Relays The Prophecy To Voldemort
Snape was in James Potter's debt, as in his fifth year Sirius thought it would be funny to lure him into the Shrieking Shack where Remus was in his werewolf form. Snape went sneaking down the tunnel by the Whomping Willow, but James grabbed him in time. When Lily confronted him, Snape maintained James was no hero but merely saved his and his friends' neck. He loved Lily deeply, and he was also aware James fancied her. In Harry Potter and the Deathly Hallows chapter 33, "The Prince's Tale," Lily called James "an arrogant toerag," but at the same time, disapproved of the company Sev kept. They continued to be friends until the fateful day when James hoisted Severus upside down which revealed his underwear. Lily came to his defense but ultimately Snape calling her the unforgivable word, mudblood led to their estrangement.
Though Snape begged forgiveness, Lily walked away saying she could no longer make excuses for his behavior and that she knew him and his friends aimed to be the Death Eaters. After finishing the seven years at Hogwarts, the Marauders, and Lily joined Dumbledore's Order of the Phoenix to fight Voldemort and his army. Lily married James and the couple had a baby boy, named Harry James Potter. They also named Sirius Black as his godfather. Snape meanwhile was Voldemort's loyal Death Eater, working at his behest. Harry Potter and the Deathly Hallows' chapter 33, "The Prince's Tale" traces how Snape earned his redemption after Lily's death and secretly allied with Dumbledore as a double agent against Voldemort.
Chapter 37 of Harry Potter and the Order of the Phoenix reveals that Voldemort's agent, Snape had overheard and relayed to him the beginning of Sybill Trelawney's prophecy while she was interviewing with Dumbledore. This prophecy foretold the birth of a boy born at the end of July to parents who thrice defied the Dark Lord. This boy would have the power of vanquishing Lord Voldemort. While the prophecy applied to Harry and Neville Longbottom, Voldemort chose Harry to be the one Sybill was referring to. Snape had only heard the beginning, as he was detected and thrown from the building, he couldn't inform his master that to attack this boy would mean transferring his powers to him and making an equal. This prophecy had a cataclysmic effect on the Potters, and it altered the course of Harry's life.
Snape Defected From The Death Eaters
Snape who back then served Lord Voldemort hurried to pass on the partial prophecy to his master. He had no possible way to know which boy would Voldemort go after and that the Potters' lives would be destroyed in the Dark Lord's murderous quest. Chapter 25 of Harry Potter and the Half-Blood Prince reveals that Snape was filled with remorse when he realized how Voldemort had interpreted the prophecy. "I believe it to be the greatest regret of his life and the reason that he returned -" says Dumbledore while establishing Snape's relative innocence. Harry Potter and the Deathly Hallows chapter 33 reveals the morally ambiguous Snape requesting his master to spare Lily's life in exchange for her son. But when he realized Voldemort was set on killing the Potters, he came to see Dumbledore. He requested Dumbledore to hide the Potters, while the latter demanded a price in return.
"Anything," said Snape, before finding out Lily and James were dead and the boy lived. He wished he were dead, but it was too late by then. It was then that Dumbledore explained how Snape could make amends for his grave mistake. He asked Snape's help in protecting Lily's son because he was certain of Dark Lord's return. From then on, Snape spied for the Order, became Dumbledore's confidant, and ultimately inherited the burden to kill Dumbledore. He redeemed himself through countless acts such as trying to unjinx Harry's broom in the first Harry Potter movie and shielding the trio from Lupin's werewolf form in the third Harry Potter movie - The Prisoner of Azkaban. He lied, spied, and put himself in mortal danger to honor his love for Lily. While yes, he did not care for Harry in the truest sense, he did protect him because he'd "always" loved Lily.