Duelo McFile serves as the ship doctor for the Nirvana, a vessel where he is one of only a few men among a crew of female pirates. His background is that of an elite citizen from the male-dominated planet of Taraak, where he was a top graduate of the military academy and holds a medical license. His reputation is such that it was said he could obtain any position he desired simply by submitting a blank resume with only his name on it, a testament to his status as one of Taraak's most distinguished individuals.

In terms of personality, Duelo is notably calm and collected, often maintaining a quiet and observant demeanor. He approaches most situations with a cool and analytical mind, which serves him well in his medical duties. However, this composure can falter when he encounters scenarios that lie outside the scope of his extensive education and experience. A prime example of this occurs when he must coach the inexperienced Hibiki and Dita through the delivery of a baby, a situation where his textbook knowledge meets the messy reality of practice, leaving him visibly awkward and confused. His curiosity is a strong driving force, and he is particularly fascinated by human anatomy, including a deep interest in the biological differences between the sexes, which are a mystery to men from his world.

Duelo's primary motivation stems from his intellectual curiosity and his dedication to his role as a physician. He takes charge of medical emergencies with quiet authority, often overriding the objections of the female crew members who are initially distrustful of a man. He assures them he is not a threat, and because the Nirvana's own medical facilities are non-functional and he is the only licensed doctor on board, the female pirates have little choice but to accept him as their official physician. His role in the story extends beyond just treating injuries and illnesses. He makes a crucial and analytical discovery concerning the heritage of the main protagonist, Hibiki, uncovering a link between their two worlds that has significant implications for the series' central conflicts.

Key relationships shape his character arc significantly. He forms a close working relationship with Parfait, a female mechanic aboard the Nirvana. She is the first woman he has ever worked closely with, and their professional partnership gradually deepens into a mutual friendship and a clear, though understated, romantic attraction. This relationship represents a bridge between the two segregated genders and marks a significant personal development for him as he moves from clinical observation to genuine personal connection. His development throughout the series is subtle but present, as he evolves from a somewhat detached analyst to a more engaged member of the crew, learning to navigate the complex social dynamics of living and working alongside women.

Duelo also possesses several notable characteristics and abilities. As a ship's doctor, his medical expertise is exceptional and vital to the crew's survival. He is physically imposing, standing 188cm tall with a corresponding build, and is seventeen years old. A distinctive and often-commented-upon visual trait is that his hair always covers the right side of his face, obscuring his right eye from view. This mystery is finally addressed at the very end of The Second Stage when the young crew member Paiway sneaks up and lifts his hair to see his hidden eye. The audience sees only Duelo's back and the pleased, contented expressions of Paiway and Parfait afterward, but what she saw and what makes his hidden visage so noteworthy remains a deliberate secret.