In the Diary of Ellen's backstory, she (already a witch by that point) befriends a boy and they become good friends, only to reject her after he accidentally sees her true sick body. Feeling betrayed, she makes the house kill him afterwards. Then it's revealed that all the invisible residents of the house are leftovers from souls the demon ate. The boy is the invisible person in the library in the game and doesn't seem to attack Ellen in Viola's body. Also, while Ellen has long ago forgotten about said boy, a part of her still remembers him somehow.
"I couldn’t particularly remember the voices of everyone I’d played with, and they all seemed to blend together. But just looking at his kitten-soft hair seemed to calm my heart."
Just the whole idea of being trapped in a house where everything's trying to kill you with no way out and no way forward but towards the one who presumably created and controls it all. The novel reveals that the monsters are people killed by Ellen in the past who just want their revenge. What makes this oddly heartwarming? Her mother and the little boy mentioned before never seem to kill her, despite Ellen being responsible for their deaths.