Cesario is an eighteen-year-old ES member of the Galactic Organization of Trade and Tariffs, holding a C-class rank and operating as a secret agent for the organization. He is visually characterized by his silver hair and matching silver eyes. His name is a literary reference, derived from the pseudonym used by the heroine in William Shakespeare's play Twelfth Night.

In terms of personality, Cesario is exceedingly taciturn and reserved. He almost never speaks aloud in public, preferring to communicate essential information by whispering into the ear of his partner, Viola, or relying on body language to convey his thoughts to those around him. This quiet demeanor is a defining trait, and the character is noted for having no spoken dialogue until the final episode of the series, where he delivers a single sentence. Despite his silence, he has a subtle stubborn side, particularly regarding personal preferences such as food, where he favors Chinese cuisine in contrast to Viola's preference for Italian food, leading to minor but recurring lighthearted disagreements between them.

Cesario's primary role in the story is as the partner assigned to Viola, forming a two-person team of GOTT agents. The dynamic of this partnership is central to his function within the narrative. While they generally maintain a strong and cooperative working relationship, his indecisive nature often makes him the losing party in their small arguments, though he quietly persists in his own desires, such that Viola frequently ends up conceding to his wishes. His ultimate function within this partnership is tied directly to his unique ability, known as Driver. This power allows him to manipulate surrounding molecules, but it cannot be activated independently. The ability is only effective when he is in physical contact with Viola, at which point he provides the necessary energy for her to exercise her own abilities. In this configuration, Cesario serves as the power source, with Viola controlling and directing the energy he provides.