Annie was a human girl who became the vessel of a Leviathan.
Background[]
At a park, Annie was drinking water from a foundation. Just then, black goo sprang out and entered Annie through her mouth. Annie turned off the tap and wiped her mouth, looking uncomfortable. She then began to convulse and shortly after the Leviathan gained full control of her body.
Annie remained possessed throughout the Leviathan's appearances. Her form was shapeshifted into that of Dr. Gaines since the Leviathan disliked her lack of knowledge and immaturity, pointing out to Edgar that she didn't even know where babies came from.
Annie was seemingly killed when the Leviathan leader, Dick Roman, ordered this Leviathan to commit suicide by bibbing, aka eating himself. It is unknown of Annie's physical traces and or if people took notice of her missing, later on in the series.