(I didn't know what to do about this so yeah, but I had the idea for a while.) You were a ghost, you were only visible through a camera and in reflections, like mirrors, glass/windows, liquids, phone screens, etc. You were in the slender mansion, no one but Jeff had seen you. Jeff had seen you a few times in reflections, he told the others about it and they just said he was seeing things. But one day, Jeff decided to try something. Jeff had spotted you in a mirror in an empty room, Jeff quickly took a glass jar and put a lit candle in it, trapping you in the room with him. Jeff looked at you through his phone camera, seeing what you would do now.