  If you have been following urlLink J. K. Rowling's site as obsessively as I have, you will know that book 6 is going to be called "The Half-Blood Prince". You will know that she is referring to neither Harry nor Voldemort. And you will have speculated.
Here's what we have come up with: * Neville (nope, he's a pureblood) * Colin (he's Muggle-born) * Dean (maybe...but don't you think he'd've had a bigger part beforehand? ) * Remus (no clue. he's a werewolf, that *could* be considered half-blood, I guess) Thankfully, nobody mentioned Sirius. Because if you had I would've been forced to hit you very hard and remind you of the Noble and Most Ancient House of Black. And of his mother burning him off the family tree for even *consorting* with Muggle-types. Then I would've hit you again.
Now, I believe I have figured out who the half-blood prince is. But first, to back up my case and increase the dramatic tension, I will quote Rowling: "Certain crucial pieces of information in book six were originally planned for 'Chamber of Secrets', but very early on (first draft of Chamber) I realised that this information's proper home was book six. I have said before now that 'Chamber' holds some very important clues to the ultimate end of the series.
Not as many as six, obviously, but there is a link. " Now I would like to remind you that the term 'half-blood' does not necessarily mean half Muggle and half wizard. I will give you a moment to digest that. And then I will present my theory: The half-blood prince is Hagrid. AHA! Hagrid is half giant. He was featured in a vague flashback in book 2. He has a very mysterious past.
It's perfect! This is my theory. My theory this shall remain. It's a good theory. 
