He is Duncan, an animatronic that will be active since Night 3. He will run trough the left or right hallway after a not-so-long path on the Cameras. If you don´t close the door on time, he will enter the office, and if you look at the opposite side or check the cameras, he´ll jumpscare you! Instead, check at him until he goes back. Close the door immediately, because he could be tricking you to check the cameras and ATTACK!
He is a spare suit of Duncan that became unused, mainly because it acted really weird... ...just look at its evil mode and you´ll see why.