According to hunter Martin Creaser they "crack open skulls and feed on brain juice". It can pass as human but its true form is revealed in a mirror.
In 5.11 Sam, Interrupted, it is disguised as a nurse called Kara. She amplifies a persons insanity to make the brain produce more chemicals like dopamine, which it likes to feed on. She feeds on the brain through a thin pointed protrusion that emerges from her wrist which she inserts into the brain.
It can be killed with Silver