New Characters:
* Regulus Arcturus Black: Harry discovers a hidden locket that belonged to Regulus, Sirius's younger brother. Regulus's actions and sacrifice play a crucial role in the story.
* Horace Slughorn: A former potions master who returns to Hogwarts to teach a new class. He holds valuable information about Lord Voldemort's past.
* Bellatrix Lestrange: A powerful Death Eater and Voldemort's most loyal follower. She plays a pivotal role in the story's climax.
* Fenrir Greyback: A werewolf who becomes a major threat to Harry and his friends.
* Draco Malfoy's mother (Narcissa): While not named, she is introduced in the book and becomes increasingly involved in Voldemort's plans.
* Katie Bell: A Gryffindor Quidditch player who is targeted by a cursed necklace.
* Cormac McLaggen: A clumsy and arrogant Gryffindor student who becomes a Quidditch player.
* Lavender Brown: A Gryffindor student who becomes infatuated with Ron Weasley.
* The Inferi: Undead beings controlled by Lord Voldemort, who play a crucial role in the story's climax.
Returning Characters with Increased Importance:
* Severus Snape: His true allegiance and motives are revealed, making him a central character in the story.
* Minerva McGonagall: She takes on more responsibility as the Head of Gryffindor and plays a significant role in protecting Hogwarts.
* Albus Dumbledore: His health deteriorates, and his backstory becomes more important.
These are just some of the new and returning characters who contribute significantly to the story of Harry Potter and the Half-Blood Prince.