Professor Horace Slughorn
Background
Horace Slughorn is a former professor at Hogwarts School of Witchcraft and Wizardry. He once had a fish named Francis who was given to him by Lily Potter. He also taught Tom Riddle and told him about what a Horcrux is.
During the events of Harry Potter and the Half-Blood Prince (2009) played by Jim Broadbent
Harry Potter and Albus Dumbledore find a house in disarray and Slughorn disguised as an armchair. Dumbledore introduces Harry to Slughorn and Slughorn tells them that the Death Eaters have been trying to recruit him and so he has been constantly moving from home to home. Slughorn shows Harry a photo he has of his former students, including one with Lily. Slughorn shows him the other photos of his collected star pupils. Slughorn then agrees to become a teacher at Hogwarts. At Hogwarts, Dumbledore introduces the students to Slughorn and tells the students that dark forces are trying to penetrate Hogwarts. Harry and Ron Weasley take Slughorn's potions class. Slughorn shows the class the love potion he made and then shows them a vial of Felix Felicis he has, or liquid luck. He then offers the vial of liquid luck to anyone who can brew a Draught of Living Death during that class day. Harry successfully brews the potion and wins the vial of liquid luck.
Later, at the Three Broomsticks, Slughorn invites Harry and Hermione Granger to join him for his dinner get together. At the dinner party, Slughorn asks Cormac McLaggen and Marcus Belby about their famous relatives. A Skinny Kid looks at Harry when Harry awkwardly stands up when Ginny enters the room, which Slughorn also notices. After the dinner, Harry asks Slughorn about Voldemort when he was at school. At Slughorn's Christmas party, Neville Longbottom, Marcus and another Waiter serve drinks. Draco is caught near the party and brought before Slughorn who is with Eldred Worple and Snape drags him outside.
Ron eats a box of chocolates that were meant for Harry and becomes infatuated with Romilda and Harry takes him to Slughorn for the cure. Afterwards, Slughorn pours a drink for Ron, Harry and himself and after Ron drinks it, he starts to convulse. Harry gives Ron a bezoar and at the hospital, Ron is taken care of by Madam Pomfrey. Slughorn tells Dumbledore that he initially intended on giving him the liquor as a gift. A few days later, Harry finds Slughorn stealing Tentacula leaves from the greenhouse. They walk over to Hagrid's hut and find Hagrid mourning the death of Aragog. Hagrid, Harry and Slughorn go inside Hagrid's hut and sing songs and drink as [[Fang}] howls. Slughorn tells the story of his pet Francis who was given to him by Lily Potter. Slughorn then gives Harry his untampered memory.