An unlikely candidate for a teaching position at Hogwarts, Horace Slughorn is a strange, hopelessly self-serving professor. Although Slughorn’s intentions are usually good, and his behavior is almost always harmless, he has extreme difficulty seeing past his own needs and desires. Occasionally, Slughorn’s greediness works out for the best. When Draco sends a bottle of poisoned Mead intended for Dumbledore, Slughorn keeps it for himself. Ron ends up getting sick from the drink but is saved by Harry, and Slughorn inadvertently prevents Dumbledore from being poisoned. Slughorn maintains a small group of potentially influential wizards (known as the Slug Club) that he romances with parties and special meetings. However, if a student is of no particular use to Slughorn, as is Ron Weasley, he gives that student no attention whatsoever. Slughorn is usually unable to remember Ron’s name, but he is quite attentive to Harry and Hermione, whom he considers important and potentially useful to him.

As later revealed in Dumbledore’s Pensieve, Slughorn was the first to tell Voldemort, then a Slug Club member and Hogwarts student named Tom Riddle, about Horcruxes. Slughorn was obviously embarrassed by this memory, as it implicated him as being far too carefree with the students he considered the most potentially powerful. Slughorn concealed and changed the memory but eventually gives up the real one, sacrificing his own reputation with Dumbledore to help Harry avenge his parents’ death.