"You shouldn't be here... :)"
Fear is a mirror/dream yokai that comes from the feelings of pain and angst built up over time along with long periods of lack of sleep. They only appear after one reaches rock bottom and lacks sleep. The chances are then increased but not guaranteed. The more days without sleep will not only increase your odds of seeing them but it also increases your odds of being killed by them if you don't sleep soon enough. At first, you will see glances of them in the mirror. As more days without sleep continue, the longer their apparances while looking into the mirror until all you see staring back is them. When it gets to that point, it's too late as the yokai will kill those that get to that point.