Mine essentially "matured" as time went on, gaining more and more attributes as I encountered more things that I found genuinely unsettling.
First off, the base. Remember the Scary Stories to Tell in the Dark series? Compilation of urban legends, with accompanying pictures far too awesome/terrifying for the audience said series was meant for. We will start with the eyeless girl, for fairly obvious reasons. Now elongate all of her limbs and fingers, so she is both colossal and incredibly gangly. Replace her teeth and lips with the distilled horror that anglerfish consider a mouth. She is never truly "still"; even when staying in place, she is constantly trembling and shaking ever so slightly. And when actually in motion she appears to be missing most of the "middle" parts of her actions, making her movement both jagged and obscenely fast.
Now the kicker: She will never, ever initiate a confrontation with you. She will instead prefer to hide in plain sight; "folding in" her huge limbs to passively observe from everyday blind spots. She is between the wall and the door as it swings inward when you enter. She is perched in an upper corner of the room, like an overgrown spider. If you make a habit of pissing into the toilet standing up, you may eventually feel her breath on the back of your neck while doing so.
And when you notice, finally notice, she will give you enough time for your brain to fully register what is happening. But not a moment more.