It's not Harry or Voldemort. JK Rowling even said so herself. and why would the book be called Harry Potter AND the half blood prince if it were Harry? The other halfbloods in the book are Crookshanks (half cat, half kneazle), Hagrid (half giant) Lupin,(half blood wizard, werewolf),Seamus ("Me dad's a muggle, mum's a witch. Bit of a shock for him when he found out"),McClaggen (a new character- J.K Rowling relased the character name at the August 2004 Edinburgh Book Festival, no one knows much about the character except for JK Rowling)or the new character described on JK Rowling's website ("He looked rather like an old lion. There were streaks of grey in his mane of tawny hair and his bushy eyebrows; he had keen yellowish eyes behind a pair of wire-rimmed spectacles and a certain rangy, loping grace even though he walked with a slight limp.") And someone mentioned Mark Evans. JK Rowling said that Mark Evans is NOT related to Lily Evans in any way.