Mr. Hornick, aka The Mysterious Man, aka Subject #0, was a character in the AP series. Initially believed to be a regular high school government teacher, it was revealed he was connected to the vast Educational Protocol conspiracy. Ultimately, he was revealed to be a clone of the real Hornick, acting as a figurehead for the organization to keep it's true creator's identity a secret.
Hornick initially appeared during the end credits of How to be a Good AP History Student 3. Not much is known about the character; no context is given for him.
In his one scene, he confronts Mike Dame and tells him to prepare for AP Government, only to be interrupted by the school AP History Teacher shouting "No!!!" The scene then cuts to black.
In real life, Mr. Hornick is the government teacher for most of the students involved in this project. As for the film, it's possible that his role may be expanded on in spin-offs; indeed, the scene itself seems to hint towards an "AP Government" related film.