Gabriel himself is nightmare fuel incarnate. While initially an enigmatic killer with a vague connection to Madison, he is revealed to be a parasitic vestigial twin with supernatural powers and had been living inside Madison for years. Before his operation, he grew malevolent and killed several facility members. Due to sharing the same brain with his twin sister, Gabriel couldn't be completely removed unless it resulted in Madison's death or permanent brain damage. Even when he was locked away in the back of her head, he nevertheless tried to coerce Madison to commit vile acts in his stead starting by telling her to cut a piece of birthday cake only for her to nearly kill her unborn sister by carving out her mother's womb.
Gabriel's face, before and after he's removed from Madison. While still having a body, he resembles a grotesque screeching parasitic monstrosity. His final form in adulthood is a malformed snarling demonic visage, coupled with his jerky body movements as he uses Madison's body to kill.
After Madison receives a brutal thrashing by the female inmates, this was enough force to awaken Gabriel. While screaming in agony, Madison holds the back of her head which splits open to reveal Gabriel's demonic face. Worse, her limbs twist around with audible cracking noises leading to Gabriel savagely massacring all the prisoners and then tearing into every police officer until two remained.
