Draco Malfoy
It is learned that Voldemort created six Horcruxes to gain immortality and that they must all be destroyed before Harry goes after the final piece of Voldemort's soul in the Dark Lord's body. Harry also repeatedly warns Dumbledore in most of their lessons that school bully Draco Malfoy is working for Voldemort. Dumbledore refuses to take any action against Draco, and instead tells Harry that he already knows more about what is happening than Harry does.