Rather belated (and possibly noted already), but in case there was any lingering doubt as to Mr. Hat-and-Cloak's identity:

(Chapter 35): Mr. Hat and Cloak gave a whispery chuckle. "Indeed," said the whisper. "With the murder of one student five decades ago being the exception that proves the rule, since Salazar Slytherin would have keyed his monster into the ancient wards at a higher level than the Headmaster himself."

(Chapter 49): Professor Quirrell sipped from his own waterglass again. "Well then, Mr. Potter, I shall freely tell you what I know or suspect. [...] Therefore [Myrtle's] murder was performed either by Headmaster Dippet, which is unlikely, or by some entity which Salazar Slytherin keyed into his wards at a higher level than the Headmaster himself."