Harry Potter and the Half-Blood Prince
How we cite our quotes:
He had never had a proper conversation with the headmaster outside of Hogwarts before; there was usually a desk between them. (4.1)
Harry and Dumbledore's friendship changes and deepens throughout the course of Harry Potter and the Half-Blood Prince. They become more like colleagues, equals. Dumbledore trusts Harry enough to let him in on all of the details of his top-secret quest to find Voldemort's Horcruxes. The two trust each other implicitly.
"Or perhaps, to confess that you yourself are worried and frightened? You need your friends, Harry. As you so rightly said, Sirius would not have wanted you to shut yourself away." (4.212)
Dumbledore knows that friendship is the key to life; we cannot exist without good friends to help us and to love us. Harry cannot carry the prophecy by himself – it would be too much of a burden. Dumbledore knows this and knows that all humans need support. Harry, Ron, and Hermione's friendship is pretty rare and deep. Voldemort does not have any friends, making him extremely different from Harry.
"A warmth was spreading through him that had nothing to do with the sunlight; a tight obstruction in his chest seemed to be dissolving. He knew that Ron and Hermione were more shocked than they were letting on, but the mere fact that they were still there on either side of him, speaking bracing words of comfort, not shrinking from him as though he were contaminated or dangerous, was worth more than he could ever tell them." (5.203)
If Harry is "pure of heart," than we argue that Ron and Hermione are too. They do not hang out with Harry in the hopes of finding great wealth, fame, and fortune. They do not always agree with Harry or support what he does. They are friends with Harry because they like him as a person, they have similar likes and dislikes, and they have fun together. Harry never once seeks out friends that will make him seem cool or popular or edgy. He stays true to himself, and, as a result, we get a glimpse of one of the greatest friendships in all of literature.