This demon is Dodge, and as we learn over the course of the season Dodge is actually their dad’s childhood friend Lucas using the Mirror Key. Confusing? Sure is. As we learn via flashbacks, when Rendell was growing up, he and his friends were the “keeper of the keys” until things went horribly wrong. Lucas became possessed by a demon after they opened the Omega Door and sought to keep all the keys for himself. In a fit of rage, he killed two other friends before Rendell killed Lucas. The group was then forced to cover up the murder, claiming that these three friends drowned after being caught in the rising tide in the sea cave. Later, this basement room in the Key House was boarded up and closed off to everyone. (Also, Rendell’s younger brother, Duncan, witnessed these murders, so the decision was made to remove his memories.)