Ceionius is a significant supporting character in Thermae Romae, primarily known as the hedonistic and ambitious adopted son of Emperor Hadrian. His position as a designated heir to the Roman throne places him at the center of the empire's political tensions, often serving as an obstacle to the wiser and more level-headed counselor, Antoninus. In the narrative, Ceionius represents a potential future for Rome that is rooted in personal indulgence rather than civic duty.

The personality of Ceionius is consistently defined by his reputation as the "number one womanizer of Rome," a trait that overshadows any political or military acumen he might possess. He is depicted as obnoxious and hedonistic, frequently seen parading with women immediately upon returning to the city and seemingly more interested in personal pleasure than the weighty responsibilities of leadership. His availability for political advancement is questionable; he is known to grope others, suggesting a casual and boundaryless approach to those around him that further cements his unflattering image. From the perspective of the protagonist, Lucius, Ceionius appears to have no obvious redeeming qualities, with his womanizing being the most memorable aspect of his character. The motivations of Ceionius are largely self-serving, centered on maintaining his status as the eventual successor to the throne. However, he is not portrayed as entirely without martial responsibility, as he is sent on an imperial mission to the frozen battlefields of Pannonia.

In the story's role, Ceionius acts as a counterpoint to the moral and competent leadership embodied by Antoninus. His very existence as the emperor's successor is a source of narrative friction, as it is Antoninus who is portrayed as the more capable and thoughtful choice for the future of Rome. The emperor's reliance on Ceionius appears to be born more from political necessity and fear than from genuine confidence, as Hadrian is concerned about his successor falling ill. This fear is grounded in historical inevitability, as a key plot point reveals that Ceionius is destined to die of the plague in Pannonia, an event that paves the way for Antoninus to inherit the throne. His key relationships are thus fundamentally antagonistic towards Lucius and Antoninus, while he maintains a familial, if perhaps strained, connection to Emperor Hadrian as his adopted son. Ceionius does not undergo significant personal development within the story's frame; he remains a consistent figure of decadence whose primary function is to highlight the virtues of his rivals and the threat of misrule that his succession represents.

Notable abilities of Ceionius are rarely mentioned in a positive light. He is explicitly not remembered for any particular strategic or architectural skill, with his lack of ability being so pronounced that even those who serve under him question his competence and character. Instead, his talents are purely social and indulgent, limited to his reputation as a relentless womanizer and a person who surrounds himself with fleeting pleasures rather than substantive achievements. His presence in the story is therefore less about his own actions and more about the political and historical weight he carries as the flawed heir whose downfall is required for the empire's stability.