If you mean the one who peeks into your office through the right door, you have to have high sanity. closing the doors does not prevent her from getting in, and even then, when she's at the window, she won't kill you if you don't close the door. I tested this in my own game, so i know its true. The only way to deal with her is high sanity, and a benefit of her being there is she actually prevents anyone outside the right door from getting in while there, but keep an eye on her so you know when she leaves, so you can close the door if need be.
If you are refering to the red one that appears on night 12, she starts in the long hallway on the far right of the map. She doesn't move when the camera is on her, so whenever free, watch her. She moves from the starting hallway, to the middle room, to the right hallway outside your office, then instakills you if she reaches the door. The best method is to just juggle her with everyone else, which is relatively easy given the shorter nights compared to other FNaF games.