This unnamed boy was the brother of the Girl in the walls.
This boy and his sister were the result of an incestuous rape by Bill Gibson upon his daughter, Rebecca.
The two siblings were forced to live inside the walls of the house, where they consumed rats and wore rags. They never studied nor went outside or learned to speak English.
One day, his sister decided to kill Bill, leading to an investigation by Sam and Dean Winchester. The siblings began harassing the Carter family, who intended to move into their house, leading Sam and Dean to fight the siblings off.
Entering through a hidden dumb waiter in the kitchen, Dean finds their weapons and Danny bound under the floor space. As he tries to free him, he is attacked by this boy. Sam and Brian manage to hoist Danny up from the basement with a lifeline made with curtains while Dean fights with the boy, whom he eventually shoots and kills.
This boy's sister attacked Susan Carter and Kate Carter where they hid outside, and Brian Carter killed her to protect them. After the case, Dean says he feels pity - and empathy - for the children who were turned murderous by a life of torture.