The Fourth Wall is a type of metaphoric "barrier" that's between videogames and the real world. Breaking the Fourth Wall is considering a character knows that they are in a video game (or any piece of media in that case). Only two characters break the fourth wall in Epic Mickey. In Epic Mickey 2, one character breaks the fourth wall.
Gus breaks the fourth wall only in cutscenes, although it could be possible that he could be talking to Mickey. Though on his chalkboard, it shows a Wii Remote, making it more likely that he's talking directly to the player.
Yen breaks the fourth wall at the begining and end of the story, for the game is just his story to the audience. Though, it would be possible that he is telling the story to another unseen character.
Prescott breaks the fourth wall once. When Mickey, Oswald and Gus exit the Gag Factory, he moves his eyes to face the screen. Since there is no one else in the Gag Factory, he is glancing at the player.