Japan’s imperial system, the oldest continuous monarchy in the world, has reached a moment of quiet tension. For the first time in generations, the future of the Chrysanthemum Throne hinges on one person alone: a 19-year-old university student who is the country’s only remaining male heir under existing law.

That heir is Prince Hisahito, nephew of Emperor Naruhito and the youngest member of the imperial family’s male line. His position has placed unprecedented attention on a young man who, until recently, lived largely outside the public eye.

Japan’s succession rules, which date back decades, allow only men from the paternal line to ascend the throne. Women are barred from inheriting the role, and female members who marry outside the imperial family lose their royal status entirely. As the family has shrunk and male births have dwindled, the system has grown increasingly fragile.

Prince Hisahito was born in 2006, an event that temporarily eased fears of a succession crisis. But nearly two decades later, he remains the only male of his generation eligible to inherit. There are no brothers, no male cousins, and no younger heirs waiting in the wings.