Professor: Defense Against the Dark Arts, Slytherin Head of House
Affiliation: Order of the Phoenix? Death Eater?
Oh, Snape. There aren't enough words in the world that can adequately express how we feel about you. But, try this on for size: HOW COULD YOU KILL ALBUS DUMBLEDORE???
We don't know much about Snape, but we are sure dying to know more. For example, is he working for Voldemort as a Death Eater? What did he do to convince Dumbledore to trust him? What did Dumbledore know about him that we don't? And why doesn't he kill Harry when he has the chance?
We have to admit that we didn't see this one coming, but Severus Snape is indeed the Half-Blood Prince. He tells Harry this as Harry tries to stop him from fleeing Hogwarts after murdering Dumbledore. He manages to block all of the curses Harry throws at him, curses that Harry learned from the Half-Blood Prince's textbook.
What does this revelation tell us? Well, first of all we find out that Snape is really smart; he's a master at Potions and can even invent his own spells. We also learn that Snape had an affinity for the Dark Arts even at a young age. We also learn he, like Voldemort, was interested in pushing the boundaries of magic. What we want to know, however, is how exactly did his textbook end up in the cupboard of Slughorn's classroom? Wouldn't that be something that you'd want to hold onto, especially if you had spent so many hours discovering shortcuts, spells, and curses the likes of which no one had ever seen or used before? That textbook is a valuable possession if you ask us, one that Snape might have wanted to hold onto.
We also want to briefly take note of the title Snape gave himself: the Half-Blood Prince. This title reminds us of the fact that Snape (the Dark Lord's right-hand man) does not come from pure wizardry stock. Just like Voldemort, Snape's mother was a witch and his father was a Muggle. Our instincts tell us that by referring to himself as half-blood, he is emphasizing and playing up his magical background rather than his Muggle background. The "Prince" part of his title we learn (thanks to Hermione) is his mother's maiden name. He pays homage to his mom and her wizard family through this nickname, and, in the process, he makes himself kind of royal. Kind of like what Tom Riddle did by calling himself Lord Voldemort.