Lord Voldemort
Although he does not appear in this book, his past and childhood are looked at. Harry and Dumbledore discover that Voldemort has made Horcruxes (objects which a wizard can put their soul inside so that they live forever). Several of the Horcruxes, including Tom Riddle's diary, have been destroyed but more remain. Voldemort also gives Draco Malfoy the mission of killing Dumbledore, but it is ultimately Professor Snape who does so, although Snape is actually working for Dumbledore.