Question: Who is Harry's nemesis in the Half-Blood Prince?

full_context: Document 1: Harry Potter and the Prisoner of Azkaban: Harry Potter and the Prisoner of Azkaban is the third novel in the "Harry Potter" series, written by J. K. Rowling. The book follows Harry Potter, a young wizard, in his third year at Hogwarts School of Witchcraft and Wizardry. Along with friends Ronald Weasley and Hermione Granger, Harry investigates Sirius Black, an escaped prisoner from Azkaban who they believe is one of Lord Voldemort's old allies.
Document 2: Harry Potter and the Chamber of Secrets: Harry Potter and the Chamber of Secrets is the second novel in the "Harry Potter" series, written by J. K. Rowling. The plot follows Harry's second year at Hogwarts School of Witchcraft and Wizardry, during which a series of messages on the walls of the school's corridors warn that the "Chamber of Secrets" has been opened and that the "heir of Slytherin" would kill all pupils who do not come from all-magical families. These threats are found after attacks which leave residents of the school "petrified" (frozen like stone). Throughout the year, Harry and his friends Ron and Hermione investigate the attacks.
Document 3: Harry Potter and the Philosopher's Stone: Harry Potter and the Philosopher's Stone is the first novel in the "Harry Potter" series and J. K. Rowling's debut novel, first published in 1997 by Bloomsbury. It was published in the United States as Harry Potter and the Sorcerer's Stone by Scholastic Corporation in 1998. The plot follows Harry Potter, a young wizard who discovers his magical heritage as he makes close friends and a few enemies in his first year at the Hogwarts School of Witchcraft and Wizardry. With the help of his friends, Harry faces an attempted comeback by the dark wizard Lord Voldemort, who killed Harry's parents, but failed to kill Harry when he was just 15 months old.
Document 4: Harry Potter and the Half-Blood Prince: Harry Potter and the Half-Blood Prince is the sixth and penultimate novel in the "Harry Potter" series, written by British author J. K. Rowling. Set during protagonist Harry Potter's sixth year at Hogwarts, the novel explores the past of Harry's nemesis, Lord Voldemort, and Harry's preparations for the final battle against Voldemort alongside his headmaster and mentor Albus Dumbledore.
Document 5: James Potter (series): The James Potter series is an unofficial sequel-series of J.K. Rowling's Harry Potter universe, written by G. Norman Lippert. The novel series centers around the character of James Sirius Potter, Harry Potter's son, a character who makes a fleeting appearance in the novel "Harry Potter and the Deathly Hallows". The series starts with his first year at Hogwarts, set one year before the end of the Harry Potter series.
Document 6: Half-Blood Prince (disambiguation): Half-Blood Prince generally refers to the novel "Harry Potter and the Half-Blood Prince" by J. K. Rowling.
Document 7: The Tales of Beedle the Bard: The Tales of Beedle the Bard is a book of children's stories by British author J. K. Rowling. There is a storybook of the same name mentioned in "Harry Potter and the Deathly Hallows", the last book of the "Harry Potter" series.
Document 8: Harry Potter and the Half-Blood Prince (film): Harry Potter and the Half-Blood Prince is a 2009 British-American fantasy film directed by David Yates and distributed by Warner Bros. Pictures. It is based on the novel of the same name by J. K. Rowling. The film, which is the sixth instalment in the "Harry Potter" film series, was written by Steve Kloves and produced by David Heyman and David Barron. The story follows Harry Potter's sixth year at Hogwarts as he receives a mysterious textbook, falls in love, and attempts to retrieve a memory that holds the key to Lord Voldemort's downfall.
Document 9: J. K. Rowling: Joanne Rowling, {'1': ", '2': ", '3': ", '4': "} ( ; born 31 July 1965), who writes under the pen names J. K. Rowling and Robert Galbraith, is a British novelist and screenwriter who wrote the "Harry Potter" fantasy series. The books have won multiple awards, and sold more than 400 million copies. They have become the best-selling book series in history and been the basis for a series of films, over which Rowling had overall approval on the scripts and was a producer on the final films in the series.
Document 10: Harry Potter and the Goblet of Fire: Harry Potter and the Goblet of Fire is the fourth novel in the "Harry Potter" series, written by British author J. K. Rowling. It follows Harry Potter, a wizard in his fourth year at Hogwarts School of Witchcraft and Wizardry and the mystery surrounding the entry of Harry's name into the Triwizard Tournament, in which he is forced to compete.
Document 11: Harry Potter and the Deathly Hallows – Part 2: Harry Potter and the Deathly Hallows – Part 2 is a 2011 British-American fantasy film directed by David Yates and distributed by Warner Bros. Pictures. It is the second of two cinematic parts based on the novel of the same name by J. K. Rowling. The film, which is the eighth and final instalment in the "Harry Potter" film series, was written by Steve Kloves and produced by David Heyman, David Barron, and Rowling. It is the sequel to "Harry Potter and the Deathly Hallows – Part 1". The story continues to follow Harry Potter's quest to find and destroy Lord Voldemort's Horcruxes in order to stop him once and for all.
Document 12: Rubeus Hagrid: Rubeus Hagrid is a fictional character in the "Harry Potter" book series written by J. K. Rowling. He is introduced in "Harry Potter and the Philosophers Stone" as a half-giant and half-human who is the gamekeeper and Keeper of Keys and Grounds of Hogwarts, the primary setting for the first six novels. In the third novel "Harry Potter and the Prisoner of Azkaban", Hagrid is promoted to Care of Magical Creatures teacher, and is later revealed to be a member of the Order of the Phoenix.
Document 13: Severus Snape: Severus Snape is a fictional character in J. K. Rowling's "Harry Potter" series. An exceptionally skilful wizard, his coldly sarcastic and controlled exterior conceals deep emotions and anguish. A Professor at Hogwarts School of Witchcraft and Wizardry, Snape is hostile to the series' protagonist, Harry, the moment he sets foot in Snape's classroom; this hostility is rooted in Harry's resemblance to Harry's father James, who bullied and harassed Snape during their time at Hogwarts. Though philosophically attracted to the Dark Arts and Lord Voldemort's ideology of wizard supremacy, Snape's love for Muggle-born Lily Evans, Harry's mother, eventually compels him to defect from the Death Eaters. Snape becomes a double-agent on behalf of Albus Dumbledore and the Order of the Phoenix, and protects Harry throughout the series.
Document 14: Harry Potter and the Deathly Hallows: Harry Potter and the Deathly Hallows is the seventh and final novel of the "Harry Potter" series, written by British author J. K. Rowling. The book was released on 21 July 2007, ten years after publication of "Harry Potter and the Philosopher's Stone" (1997), by Bloomsbury Publishing in the United Kingdom, in the United States by Scholastic, and in Canada by Raincoast Books, ending the series that began in 1997 with the publication of "Harry Potter and the Philosopher's Stone". The novel chronicles the events directly following "Harry Potter and the Half-Blood Prince" (2005), and the final confrontation between the wizards Harry Potter and Lord Voldemort, as well as revealing the previously concealed back story of several main characters. The title of the book refers to three mythical objects featured in the story, collectively known as the "Deathly Hallows"—an unbeatable wand, a stone to bring the dead to life, and a cloak of invisibility.
Document 15: Harry Potter (character): Harry James Potter is the title character and protagonist of J. K. Rowling's "Harry Potter" series. The majority of the books' plot covers seven years in the life of the orphan Potter, who, on his eleventh birthday, learns he is a wizard. Thus, he attends Hogwarts School of Witchcraft and Wizardry to practise magic under the guidance of the kindly headmaster Albus Dumbledore and other school professors. Harry also discovers that he is already famous throughout the novel's magical community, and that his fate is tied with that of Lord Voldemort, the internationally feared Dark Wizard and murderer of his parents, Lily and James.
ground_truth: Lord Voldemort