(no subject)
Mark Evans.
If it's a half blood, the person has to be half Muggle. Mark's surname, Evans, is a Muggle surname -- it's the maiden name of Lily and Petunia.
Where'd the kid come from? Well, who's to say Lily and Petunia didn't have relatives with the surname Evans. I've got tons of relatives with my surname.
Yeah, yeah, I know, Dumbledore said in the first book that Harry had no other living relatives besides Petunia. But, he needed to be placed with the Dursleys for a number of reasons, so Dumbledore could have fudged a bit.
Plus, JKR tends to mention characters before they become important. She mentioned Sirius in book one, and she mentioned the Lovegoods in book four.
I'm probably way off, but I thought I'd post anyways.