I think it might be one of three people.
-A new person that hasn't appeared yet.
-Wormtail (Dumbledore hinted something in the 5th book about him)
-Seamus Finnigan
The new person is very likely to be the HBP, and I doubt that Wormtail is it. As I read the answers, I noticed that not many people remember the 1st book all too clearly (and those who only have seen the movie would not have a clue).
I remember Seamus Finnigan saying in chapter seven, book 1, page 125, 3rd paragraph, stating, "I'm half-and-half. Me dad's a Muggle. Mom didn't tell him she was a witch until after they were married. Bit of a nasty shock for him."
See my point? I think it's most likely to be Seamus.