The "American Slow-Walking Creepy Girl" following a wounded soldier.
The American Slow Walking Creepy Girl is one of the monsters seen in the facility during the Purge. She can be seen in the background directly after Dana and Marty flee the Elevator Room as the Dragonbat pursues them. She is not listed on the whiteboard and has a similar concept to the Japanese Floaty Girl.
Physical Description & Attributes
She is seen as a young woman with stringy, unkempt black hair. Her dress seems to be from a hospital.The only time she is seen during the film is in the distance, walking slowly and singing "Hush, Little Baby" towards a wounded security guard, presumably about to obliterate him. She wears a white hospital gown and is barefoot.
She walks slowly toward a door in the background singing as Marty and Dana wait to see if the Dragonbat will reemerge from a massive hole in the wall it makes when it drags a scientist into it. Before she sings, she questions "Where are you going?" to the injured guard.
It is unknown what item in the cellar summons her.
Inspiration
She seems to be a reference to Alma Wade, a major antagonist in the F.E.A.R video game series who is the ghost of a powerful psionic. Indeed, the scene in which she appears mirrors a scene from the games. It is probably safe to say that the Slow-Walking Creepy Girl is a ghost, possibly with psychic abilties like her inspiration.
Her horror character archetype is also somewhat reminiscent of Samara from The Ring.