Nicolas Thomas is a firefighter employed as part of the security services at the Louvre Museum in Paris. He appears in the story Rohan at the Louvre, which follows the manga artist Rohan Kishibe as he investigates a legendary cursed painting hidden in the museum's depths. In the original manga one-shot, Nicolas Thomas and his colleague Hugo Renard serve as tertiary allies, assisting Rohan and the museum staff during their expedition into an abandoned underground storage area. In the live-action film adaptation, their role is shifted to that of tertiary antagonists, part of a criminal group involved in a scheme to replace valuable artworks with forgeries.

Nicolas Thomas is characterized primarily by his professional demeanor as a security firefighter. He operates with a focus on safety protocols, such as asking the exploration party to surrender any flammable items or objects that could damage stored artwork, including lighters, pens, knives, and keys. This attention to regulation reflects a background in institutional safety procedures rather than any personal interest in art or the supernatural. His motivations in the story are tied to his duty and, in the film version, to his involvement in the forgery operation, which positions him as an obstacle to Rohan's investigation rather than a helper.

Within the narrative, Nicolas Thomas accompanies Rohan, his editor Kyoka Izumi, curator Ryunosuke Tatsumi, and interpreter Emma Noguchi into the sealed Z-13 warehouse. The group enters the maze-like, disused archive where the cursed painting is hidden. His presence is functional, providing logistical support for moving through the hazardous environment. In the adaptation, he joins the antagonist group that attempts to deceive Rohan and replace the museum's genuine pieces with counterfeit works created by the painter Morris Legrand. This criminal involvement defines his antagonistic turn, contrasting with the more neutral ally role seen in the original comic.

His key relationships are largely professional. He works alongside Hugo Renard, his fellow firefighter and partner in the security detail. Together they interact with Rohan, Tatsumi, Emma, and Kyoka as part of the expedition team. In the film, this partnership extends into the forgery conspiracy, making the two firefighters co-conspirators with the curator and the forger. Nicolas Thomas does not share a personal or developing bond with Rohan; their connection is limited to the brief, tense encounter in the underground vault.

Nicolas Thomas has minimal character development. He appears only during the Louvre sequence and does not have a backstory or arc of his own. His role is to facilitate the plot's progression, either by enabling access to the restricted area or by providing a physical threat during the conflict that erupts when Rohan exposes the conspiracy. In the film, his death at the hands of the painting's supernatural curse serves as a narrative consequence for his criminal actions, but no personal growth or change is depicted.

Regarding notable abilities, Nicolas Thomas is an ordinary human with no supernatural powers. His skills are those of a trained firefighter and security officer: knowledge of the museum's layout, familiarity with safety equipment, and the physical capability to handle emergencies. In the confrontation with Rohan, he relies on brute force and intimidation rather than any special talent. His effectiveness as an antagonist comes from his position within the museum's infrastructure and his willingness to use violence to protect the forgery operation, not from any unique combat or investigative expertise.