Niels Bohr is a world-renowned, half-Jewish Danish physicist widely regarded as the father of modern atomic physics. Operating much like a pope within the scientific community, he hosts numerous brilliant assistants in Copenhagen. He approaches physics methodically, acting as a slow and careful thinker who concerns himself deeply with the moral paradoxes of scientific discovery. His status in occupied Denmark puts him at constant risk.

Werner is a brilliant German physicist working under the surveillance of the Gestapo during the Nazi regime. Once Bohr's closest assistant and an adopted son figure to the Bohrs, he now conducts atomic research for the German state. Unlike his former mentor, he races rapidly toward mathematical conclusions, sometimes overlooking the broader consequences of his scientific pursuits.

Margrethe is Niels Bohr's wife and a highly capable intellectual partner who spends hours typing and editing his scientific papers. She possesses sharp emotional intelligence, allowing her to detect the personal flaws, competitive arrogance, and unspoken tensions between her husband and his colleagues. She serves as a grounding force in the room, cutting through dense scientific jargon to focus on plain truths.

Christian is the son of Niels and Margrethe Bohr who died in a 1934 boating accident. His tragic death, during which Niels was powerless to save him, left a permanent emotional void in the family and heavily influences Niels's feelings of helplessness in other crises.

Schrodinger is a fellow physicist who competes fiercely with Heisenberg for academic positions and scientific breakthroughs. Their bitter rivalry highlights Heisenberg's relentless, combative nature in academic circles, as the two argue constantly.

Harald is another of Niels and Margrethe Bohr's children. He passed away at a young age due to medical issues, adding to the profound sense of loss that hangs over the household.

Georg Duckwitz is a German embassy worker with close ties to Heisenberg. He plays a background role in the complex political reality of occupied Denmark, demonstrating the quiet ways information moves during the war.