Lucio Modesto, known in Latin as Lucius Quintus Modestus, is the protagonist of Thermae Romae. He is a Roman bath architect and engineer active during the reign of Emperor Hadrian in the 2nd century AD. His family has a long tradition of designing public baths, and he studied architecture in Athens. Lucio is a proud, patriotic Roman citizen who holds deep respect for traditional Roman bath culture. He is extremely earnest, hardworking, and meticulous in his craft, often to the point of stubbornness. His conservative approach to design causes him to fall out of step with the fashionable, novelty‑driven tastes of his era, and he loses his job at an architectural firm early in the story.

Lucio’s primary motivation is his passion for creating high‑quality, functional baths that serve the Roman people. He constantly seeks to improve his designs but struggles to generate fresh ideas. This creative block is resolved when he unexpectedly slips through a temporal passage while submerged in a public bath and finds himself transported to modern‑day Japan. There he encounters Japanese bathhouses (sentō), hot springs, and advanced toilet technology. Initially bewildered and nicknaming the Japanese “flat‑faced slaves,” he quickly becomes fascinated by their innovations: fruit‑flavored milk, bidet toilets, outdoor rotemburo, stone spas, and bathing etiquette. He brings these ideas back to ancient Rome, adapting them with the materials and knowledge available in his own time. Although his reproductions are often imperfect, their originality earns him widespread acclaim and eventually the patronage of Emperor Hadrian himself.

Lucio’s role in the story is that of a curious cultural translator and an inventive problem‑solver. Each episode or chapter follows his accidental journeys between Rome and Japan, where he learns something new about bathing culture and then applies it to solve a challenge in his own era—whether it is healing soldiers, improving an inn’s service, or building a private bath for the emperor. His time‑travel is involuntary and triggered whenever he is in a dilemma and submerges himself in water.

Key relationships include his close friend Marcus Pietras, a cheerful marble sculptor who often helps Lucio realize his projects and provides emotional support. Emperor Hadrian becomes a significant patron, commissioning Lucio to construct baths in his villa and later entrusting him with larger projects. Hadrian’s admiration for Lucio’s work grows throughout the series. In the later parts of the story, Lucio meets and marries Satsuki, a Japanese researcher of ancient Roman history, with whom he has a son named Marius. This cross‑cultural relationship deepens his understanding of both worlds.

Lucio undergoes notable character development. He begins as a frustrated, traditionalist architect on the verge of obsolescence, but his encounters with Japan broaden his perspective and revive his career. He learns to appreciate innovation without abandoning his core values, and he becomes more open‑minded about foreign cultures. However, his intense dedication to work sometimes strains his personal life, costing him an earlier marriage and leading to moments of loneliness.

Among Lucio’s notable abilities are his deep knowledge of Roman engineering, his skill in adapting foreign concepts to ancient technology, and his ability to inspire others with his creations. He is also physically resilient, often appearing naked or in a towel during his time‑travels, and he has a knack for observing details that others overlook. His most distinctive trait is his unwavering pride in being Roman, which provides both comedic contrast and a genuine sense of cultural curiosity.