Brick is a Troll, born and raised in Ankh-Morpork. Since trolls are made of metamorphorical rock, he has taken on the characteristics of a typical Ankh-Morpork material and is built like a brick wall or, as is noted in Thud!, the proverbial brick shithouse. He shares his make up of fire hardened clay with golems so perhaps that explains his ability to survive in spite of what he is doing with Troll drugs to fry his brain. He witnessed the murder of Grag Hamcrusher, but, being high on Slab at the time was not immediately aware of what he was seeing. It is believed that he found himself in the Dwarf tunnels accidentally, while attempting to find a good place to enjoy his drug-induced hallucinations.
Before his patronage by Detritus, Brick was a loser's loser. Other trolls ignored and avoided him when possible, as real people ignore and avoid drug addicts and the homeless, and even those other lowlifes around him barely put up with him enough to let him scrounge Slab and not throw things at him. The most common sentiment he encountered was to be called a "sedimentary coprolite". He had next to zero self-esteem, himself vaguely aware that he had sunk so low that even the cockroaches spat on him.
Detritus drew attention to the fact that Brick is tougher and stronger than anyone has given him credit for. As well as surviving life on the streets as an unloved gutter troll, he can still be fairly rational, by troll standards, after much determined auto-frying of his brain on scrape: and most of all, he can still stand and fight, badly hurting Samuel Vimes himself, after taking a skinful of luglarr. These are qualities of underlying strength and resilience that a tough street-wise troll like Detritus may even admire in some way, and which influenced his decision to take a stern and loving paternal hand, or rather fist, in the upbringing of young Brick.