This page lists characters whose identity is unknown or uncertain at some point.
He reveals himself to be Tom Riddle / Voldemort in Chapter 104.
The Game ControllerEdit
In Chapter 13, Harry plays The Game without knowing who the Game Controller is. The Game Controller is revealed to be Harry himself in Chapter 14.
Harry finds notes on his bed in Chapters 14, 21, and 63, that are signed Santa Claus. Dumbledore later claims to have written them (Ch. 79).
In Chapter 50, Padma Patil is approached by someone invisible. She first correctly suspects him to be Harry Potter, but he half-denies it. She will later think that she spoke to the ghost of Salazar Slytherin (canon) (Ch. 71).
In Chapter 77, Hermione is approached by someone wearing a black cloak and a black hat, who then turns into a shining lady wearing a white dress and a white veil. While not explicitly revealed, it is suspected to be Quirrrell initially trying to pit Potter against Dumbledore via Zabini and later attempting to tempt Hermoine. It is the moment he starts tampering with her mind, ultimately leading to the attempted murder of Draco. This is evidenced by the headaches and fatigue Hermoine experiences while talking to Mr. Hat-and-Cloak.