Marcus Pietras is the best friend of Lucius Modestus, the central protagonist. He works as a professional sculptor and stonemason. His expertise in stone carving leads to frequent collaboration with Lucius; Marcus carves the stonework for Lucius's bathhouse construction projects. A specific challenge arises when Marcus struggles to meet deadlines, including commissions for Emperor Hadrian, because he must regularly escort his master to public bathhouses. Observing this time constraint, Lucius designs and builds an indoor bath specifically for Marcus's master. This solution frees Marcus from the need for frequent public excursions, granting him more time for sculpting work. Beyond their professional partnership, Marcus and Lucius share a personal friendship. They socialize together, including visits to bathhouses where Marcus clearly enjoys the leisurely atmosphere and available amenities, such as hair removal treatments. Marcus Pietras's established role remains consistent: he functions as Lucius Modestus's supportive friend and professional associate within the narrative.