Ryusui Nanami is a major character introduced in the later stages of the story as an essential ally to the Kingdom of Science. He is the son of the owner of the Nanami Conglomerate, a massive financial empire, and grew up in an environment of immense wealth and privilege. From a young age, he was obsessed with the sea, spending enormous sums of money on ship models before progressing to building his own vessels and successfully circumnavigating the globe on a self-made yacht while still in middle school. These experiences honed him into an expert sailor and navigator, which becomes his primary contribution to the team.

Ryusui is defined above all else by his all-consuming desire and ambition. He openly proclaims that his "desire is justice" and that he wants to possess everything in the world, a philosophy that earned him the title of the greediest man in the world both before and after the petrification event. His personality is flamboyant, charismatic, and incredibly confident, often punctuated by a loud laugh and the dramatic snapping of his fingers. Despite his seemingly selfish nature, his greed is not malicious; he believes that owning the world's splendor also means redistributing it for everyone's benefit. He is a firm believer in the power of currency and capitalism as a tool to motivate people and organize society, immediately inventing his own currency, the "Drago," upon being revived.

Ryusui's primary motivation is to satisfy his own desires, which luckily for the Kingdom of Science, includes a deep yearning to explore the world and eventually travel to space. He is not driven by altruism but by the thrill of the new and the challenge of acquiring what he wants. This makes him a powerful, if self-interested, driver of progress. His role in the story is multifaceted. He serves as the captain of the Perseus, the Kingdom of Science's massive ocean-spanning ship, and is an irreplaceable navigator whose skills in reading weather, wind, and currents are almost supernatural. He is also named one of the Five Wise Generals, Senku's top advisors, and is a key member of the crafting and development team, often helping to design and refine large-scale engineering projects. Beyond his practical skills, he functions as a financial strategist, introducing the concept of money and trade to the primitive world, which helps streamline resource acquisition and labor.

His most important relationship is with Senku Ishigami. The two share a deep mutual respect, with Senku recognizing Ryusui's unparalleled skills and Ryusui admiring Senku's even greater deviousness and intelligence. Ryusui once stated that in the old world, he would have tried to hire Senku for his company. While they sometimes clash over methods, they work together with a shared, almost gleeful excitement when tackling a new project. Another key relationship is with his butler, Francois, whom he specifically requests to be revived for their unparalleled culinary and household management skills. Ryusui holds Francois in high regard and notably does not care about their gender, considering it unimportant. He also has a warm relationship with the other members of the Kingdom of Science, often showing a protective side, such as when he kicked the young Suika off his ship to save her from being petrified. He is a notorious flirt, declaring his love for all women and men, finding beauty and charm in every person regardless of age or appearance.

Throughout the narrative, Ryusui undergoes notable development. Initially presented as a spoiled rich boy, his interactions with the team reveal a more collaborative and selfless side. While still greedy, he learns to channel his desires toward group goals. He also demonstrates significant resilience and courage. He is petrified and depetrified more times than any other character, becoming the first person to be revived twice and then three times, and he endures these traumatic events without losing his flamboyant spirit. A key moment of personal development comes when he volunteers to be the pilot for the moon mission, only to later realize that another character, Stanley Snyder, is more qualified. Despite his immense personal desire to go to space, he steps aside for the good of the mission, showing a capacity for self-sacrifice that contrasts sharply with his earlier image of pure selfishness.

Ryusui possesses a wide array of notable abilities. As a navigator and sailor, his skill is considered a five out of five, and he can sense minute changes in temperature, humidity, and wind direction with his body. He is a talented shipwright who can spot flaws in a design and even create perfect scale models for construction. He has a sharp business acumen and is a master negotiator. Beyond the sea, he is a quick learner and a natural pilot, capable of controlling advanced machines like drones and aircraft with little instruction. In essence, Ryusui Nanami is the engine of ambition and maritime expertise that propels the Kingdom of Science toward the new world and beyond.