Sampson Brass practices as an attorney from Bevis Marks in London. He conducts fraudulent legal work for the primary antagonist Daniel Quilp, a moneylender to Nell's grandfather. Brass executes Quilp's schemes to seize the grandfather's property through debt manipulation and aids the pursuit of Nell and her grandfather after they flee London.

He habitually cheats clients while feigning concern for their welfare. Brass employs deception, including falsely accusing Kit Nubbles of theft to discredit those helping Nell. He works closely with his sister, Sally Brass, who serves as his law firm partner, clerk, and confidante in their joint ventures. Sally notably overshadows her brother in cunning and legal sharpness.

Brass harbors intentions to marry Nell despite the significant age difference, an objective aligning with Quilp's broader plan to control the family assets. His personality mixes sycophantic behavior toward Quilp with underlying cowardice and moral weakness. Descriptions note his cringing manner, harsh voice, and unnaturally forced smiles, contributing to his repellent demeanor. The name "Sampson Brass" creates an ironic contrast, evoking strength and durability that contradict his spineless, corrupt nature.

Brass remains a consistent figure of villainy throughout the narrative, concluding with arrest and imprisonment due to his deep involvement in Quilp's criminal activities.