The Caul is a shared sentience that resides in a smoky, caul-like facial covering. Once a person is "cauled," the Caul sentience takes control, "possessing" the person, as it were. While in control, the Caul can access the host body's memory and rifle through it at will. Only through the death of the host body can the Caul sentience "free" itself to possess another.
The Caul works for Toyo Harada.