In each Harry Potter book, the 'bad guy' seems to make their appearance in Chapter Thirteen!
Sorcerer's Stone - Professor Snape threatens Professor Quirrell, who we think is a decent man for resisting Snape. We soon discover that Professor Quirrell has joined with Lord Voldemort.
Chamber of Secrets - We learn about T.M. Riddle, which stands for Tom Marvolo Riddle, and is an anagram for I Am Lord Voldemort. We thought at first that Tom was a good person because he led us to believe that he had captured Hagrid with the creature from the Chamber of Secrets.
Prisoner of Azkaban - Wormtail, or Peter Pettigrew, fakes his death by hiding, making everyone believe that he had been eaten by Hermione's cat, Crookshanks. Of course, he's a rat, so how could we have thought there could possibly be anything wrong with him?
Goblet of Fire - 'Mad Eye' Moody, or at least, who we think is 'Mad Eye' Moody, makes his appearance, and turns Draco Malfoy into a white ferret, leading everyone to believe that he's a cool guy. A little Veritaserum, and short supply of Polyjuice Potion help reveal the truth!
Order of the Phoenix - Harry has detention with Dolores Jane Umbridge, and at no point during the book could anyone have thought that she was even remotely close to being a nice, caring, or loving person.
Half-Blood Prince - Harry and Professor Dumbledore go into the Pensieve and see the first meeting of Professor Dumbledore and Tom Riddle (who is already showing his 'Dark Lord' tendencies) at the orphanage.
You know, Minister, I disagree with Dumbledore on many counts... but you cannot deny he's got style...
Phinneas Nigellus Black Order of the Phoenix, Chapter 27, Page 623
Demelza Robins, the Gryffindor Chaser in Harry Potter And The Half-Blood Prince, is named after Daniel Radcliffe's favourite charity: the Demelza House Children's Hospice, which cares for terminally ill youngsters in Kent, East Sussex and South London.