The Brethren took the boy away from his homestead in the night and allowed him to be bitten by the Lupine Wavelength Haemovariform they worshipped. He became infected by the Haemovariform that "carved out his soul and sat in his heart". The Host was given all-black eyes and was entirely controlled by the Haemovariform, which could access the Host's memories and use some basic telepathy.
Every full moon, he would transform into a true Haemovariform, the process of which was incredibly painful. The real boy was long dead. When the Tenth Doctor used the Torchwood House light chamber with the Koh-i-Noor to destroy the Haemovariform, the Host began to live again. He asked the Doctor to increase the effect and destroy him, which he did. The Host died sacrificing his life to save the world. (TV: Tooth and Claw)
The Host was somewhat sadistic — the monster claimed to have carved out his soul to enter him. The Haemovariform also appeared to be very perceptive of people and recognised that Rose was intelligent but also detected the Bad Wolf inside of her. When the Haemovariform was destroyed, he began to live again but chose to die. (TV: Tooth and Claw)