Lucius, whose full name is Lucius Quintus Modestus, is the central protagonist of Thermae Romae. He is a Roman architect and engineer living in the 2nd century AD during the reign of Emperor Hadrian, specializing in the design and construction of public bathhouses, known as thermae. Raised in a family with a strong legacy in bath architecture, Lucius lost his father at a young age and was inspired by his grandfather, who taught him that public baths are vital to keeping Roman citizens happy and content. This sense of purpose drove Lucius to follow in his father's and grandfather's footsteps, eventually studying in Athens to refine his skills.

Lucius is characterized by his earnest, serious, and deeply proud nature. He takes his work with utmost sincerity and is driven by a strong sense of duty to his craft and to Rome. However, this same seriousness often leads to frustration when his traditional designs are deemed outdated in a rapidly innovating era. He is highly intelligent and curious, with a keen eye for architectural detail. Despite his professional competence, he can be rigid in his thinking, initially resistant to change until his involuntary journeys to modern Japan open his mind. His pride as a Roman is frequently challenged by the advanced bathing culture he discovers, yet he approaches each new discovery with genuine wonder and enthusiasm, absorbing ideas with the earnestness of a scholar.

The core mechanism of Lucius's journey is an involuntary time-slip: whenever he is in a dilemma and submerges himself in water, he is transported to 21st-century Japan, landing in various bathhouses, hot springs, or bathroom settings. Initially believing the Japanese people are flat-faced slaves from a conquered land, he quickly becomes fascinated by their advanced bathing technologies, such as fruit-flavored milk, indoor bathtubs, bidet toilets, hot stone spas, and onsen culture. Unable to communicate verbally, he relies on observation and imitation. Each time he returns to Rome, he attempts to recreate these innovations using the limited materials and knowledge of his era, often producing imperfect but ingenious adaptations that earn widespread admiration.

Lucius's role in the story is that of a cultural bridge, albeit an unintentional one. His repeated time-travel serves as a comedic and educational vehicle to contrast ancient Roman and modern Japanese bathing customs. His designs, inspired by Japan, solve practical problems for Roman citizens and aristocrats alike, from building an outdoor bath with a view of Mount Vesuvius for the consul Lepidus to constructing a private bath in Emperor Hadrian's villa and a hot stone spa for Roman soldiers in Judea. His successes restore his reputation and lead to commissions from the highest levels of Roman society. He serves the emperor directly, accompanying Hadrian to Jerusalem, but remains a humble craftsman at heart.

Key relationships in his life include his friend Marcus Pietras, a marble sculptor who often carves Lucius's projects; Emperor Hadrian, who becomes a patron and admirer of his work, relying on Lucius's creations to win public favor; and his wife Livia, who supports him but eventually leaves him due to his prolonged absences and obsessive dedication to his work. This marital conflict forms a significant personal arc, as Lucius must balance his professional ambition with his personal life. His relationship with the Japanese people is also notable, though non-verbal; he views them with initial condescension that gradually shifts to awe and respect.

Over the course of the narrative, Lucius undergoes clear development. He begins as a struggling architect, frustrated by his inability to innovate. His accidental discoveries in Japan transform him from a rigid traditionalist into a resourceful and celebrated innovator. He matures in his understanding of service, learning from the hospitality of a ryokan in Edo-period Japan and the etiquette of public baths. His most significant growth involves his relationship with Livia. After building a spa town to appease the Roman populace, he reunites with her when Emperor Hadrian brings her to him, realizing that his work should not come at the cost of his family.

Lucius's notable abilities lie primarily in his architectural and engineering expertise. He is a skilled designer and builder of bathhouses, capable of surveying land, managing construction, and adapting materials. Beyond technical skill, he possesses remarkable observational intelligence, able to analyze modern Japanese facilities and replicate their underlying principles using Roman methods. He is also resilient and resourceful, able to find solutions even in hostile situations, such as when he builds a spa town for bandits who ambush him. His fearless curiosity and willingness to experiment, despite cultural shock and repeated misunderstandings, make him an effective if unwitting innovator.