Allright, for all of you that don't know this yet the half blood prince is not Mark Evans. Actually, (as sad as I am to say it) Mark Evans isn't even going to be a major character in the books. According to JK Rowling on her website Mark Evans is a nobody, it's just a coincidence he has the same name as Lilly and Peutunia, and he was just a character she needed for a walk on role. -Sigh- I know.
But anyways... I have no idea who the half blood Prince could be. We obviously need to role out Harry and Voldemort(but not necessarily Tom Riddle)Because JK Rowling said so. We also need to role out the purebloods and girls. My gut feeling is Tom Riddle.(oh and to clarify someones comment, Tom Riddle was killed in the beggining of the 4th book, but that Tom Riddle was Voldermorts Father. Voldemorts father was a muggle and his mother was a witch. When his father found out he left his wife and went to live with his parents. Then when voldermorts mother died voldermort was put in the orphanage.It became one of voldermorts goals to kill his father,the muggle he was named after. Then once voldermort became older he went to the Riddle house and killed his father and grandparents-that's the scene we see in the beggining of the book). I've also though Seamus or Dean. But I can't wait till the book comes out to find it out.