The Beast was a monster that worked as a torturer in the Doom Room.
The Doom Room
The Beast was the one and only torturer in the Doom Room. He seemed like a huge, evil monster. The Doom Room had several weapons he used to torment children who came to the Doom Room, sometimes coming to extreme measures.
The Beast was first mentioned and introduced when Sophie was sent to the Doom Room. He used Sophie's cascading, golden-blonde hair as a weapon against her. Easily figuring out her appearance was one of Sophie's biggest concerns, the Beast cut off a large amount of her long hair until it was as short as a boy's hair might be. This is where one of Sophie's first signs of being evil comes, as she, unforgiving, drowns the Beast as revenge.
Nobody is shown to ever find out where the Beast had gone or who killed him, but they are shown searching for him at times. Thinking of the Beast triggers painful memories for Sophie, who often refuses to admit how evil she is and was when she killed him. She often goes delirious and hallucinates when being reminded of the Beast.