He is a warden in the Death Row place of the prison and the second in command there. He is loyal to Paul and hates Percy as much as he does. Despite his nickname and his size, Brutus rarely hurts the inmates and is rather soft-hearted, although his temper is rather short.
However, Brutus cannot put up with disrespect and is furious when Percy mistreats the inmates. This is shown a few times throughout the movie, such as when Percy degrades Bitterbuck's body, when he says that the Green Mile is just "a bucket of piss to drown rats in" (thus referring to the inmates as "rats"), and when Percy botches Del's execution. After Percy does the latter, Brutal loses his temper and punches Percy, but Paul holds him back.