I am hoping that the Half-Blood Prince is Hagrid. Hagrid is half human and half giant (although how his parents pulled that one off . . .
). Whenever Hagrid is mentioned, it's in bits and pieces. His mother was a very important Giant who is now missing. In the last book, Dumbledore keeps mentioning how important it will be to have the Giants on their side. Hagrid was also one of the main characters in CoS. Remember that Hagrid was the person originally blamed for releasing the monster on the students (even though that was all wrong)? I'm not sure if I'm getting the books mixed or not, but wasn't Hagrid sent to Azkaban for a short term in CoS? Hagrid also can use magic, but he is not allowed to because of the incident with Tom Riddle and the mysterious monster.
Did Dumbledore keep Hagrid on the grounds and give him a job just out of kindness or because he knew his potential? I can not remember for every teacher, but it seems that Dumbledore likes to keep people around for a purpose. The main teachers you meet in the first book helped to protect the Sorcerer's Stone in one way or another. Others are members of the Order of the Pheonix. Snape was a spy because of his ties with the death-eaters. Professor Trelawney was the one who told the original prophecy about Harry and Voldemort. I'm sure there's more to Firenze's appointment than just his clan booted him. Where's Hagrid's big purpose? It could be that he's the tie between the Giants and wizards, but I think there's more to it than that.
On a side note:
The spell that hit Sirius was just a Stun spell. He was stunned and tripped when he was knocked backwards, therefore falling through the veil.
About Kreacher:
Maybe Dumbledore employs him at Hogwarts, since that's where homeless house elves seem to be flocking.
There he can keep an eye on him as well. Kreacher knows too much about the Black family and the Order of the Pheonix to be allowed to live anyone else.