Marjorie is the only child of Charles William Post and heir to the Post Cereal Company. Raised with the belief that she is as capable as any boy, she possesses a keen eye for business and a talent for high-society hospitality. Her immense wealth allows her to navigate the American elite, though she often feels out of place among old-money families and struggles to balance her ambition with societal expectations of women.

Charles William is Marjorie's father and the founder of the Post Cereal Company. After recovering from a debilitating illness through alternative diets and spiritual healing, he builds a massive fortune creating healthy, easy-to-prepare foods like Grape-Nuts and Postum. He teaches Marjorie about business and finance from a young age, treating her as his primary heir.

Ed is a young law student from an established, wealthy Connecticut family. He is drawn to Marjorie's energy, though he is highly conscious of class distinctions between his inherited wealth and her family's commercial fortune. He prefers a traditional domestic arrangement and often disapproves of Marjorie's active public life.

Ned is a self-made Wall Street millionaire who shares Marjorie's energetic approach to business and philanthropy. He takes an active role in the Post Cereal Company, eventually serving as chairperson of the board. Though he enjoys the lavish lifestyle Marjorie's fortune affords, he strongly prefers to maintain final authority over corporate decisions.

Joe is a confident, politically connected lawyer from Wisconsin who receives an ambassadorship from the Roosevelt administration. He is highly invested in international diplomacy and relies on his political friendships to maintain his status in Washington. His career takes him to the Soviet Union, placing him near the center of global events.

Herb is an independently wealthy widower who crosses paths with Marjorie later in her life. He shares her interest in philanthropy and enjoys a comfortable high-society lifestyle, splitting his time between Washington, DC, and Pittsburgh.

Alice is the famously freewheeling eldest daughter of President Theodore Roosevelt. She lives an unconventional, highly publicized life and ignores the strict expectations placed on women of her social standing. She serves as a lifelong friend to Marjorie, offering blunt advice about men and high society.

Ella is Marjorie's mother, a woman who finds solace in the teachings of Christian Science during her husband's health crisis. She prefers a quiet, spiritually grounded life and struggles to adapt to the massive wealth and shifting dynamics brought on by the success of the Post Cereal Company.

Leila is an employee of the Post family who manages Marjorie's early education and assists Charles William with his growing business. Her constant presence in the household eventually causes significant friction between Marjorie's parents.

Adelaide is Marjorie's eldest daughter. Raised amid immense wealth, she grows up traveling between her mother's various luxurious estates and navigating the expectations of high society.

Eleanor is Marjorie's middle daughter. She develops an independent streak that occasionally causes friction with her mother's social expectations and strict rules regarding wealth.

Nedenia, often called Deenie or Dina, is Marjorie's youngest child. She frequently travels with her parents on their yachts and later volunteers for the USO to support the military during wartime.

Harcourt is Ned Hutton's college-bound son from a previous marriage. Marjorie welcomes him warmly, hoping to integrate him smoothly into her established family life.

Mrs. Gregory is a widow and Christian Scientist who hosts the Posts during Charles William's illness. She introduces the family to her faith and her home-cooked meals, directly defying the local sanitarium's strict rules.

Dr. Kellogg is a rigid physician who treats Charles William's illness with highly regimented dietary restrictions. He invents early forms of cereal and peanut butter, inadvertently inspiring the Post family's future business empire.

Colby is a loyal friend to the Post family and serves as the president of General Foods. He acts as a steadying presence in Marjorie's life and supports her expanding role on the company's board of directors.

Madame Molotova is the wife of the USSR Premier. She is deeply embedded in the Soviet political system and views Marjorie's commitment to American capitalism with pragmatic calculation, famously selling her confiscated tsarist treasures.

The Premier of the USSR acts as a powerful leader within the Communist Party. He navigates complex international relationships on the brink of World War II, accepting American hospitality while maintaining strict Soviet alliances.

FDR is the President of the United States, managing the country's response to rising global conflicts. He relies on personal connections and trusted allies to maintain diplomatic channels with foreign powers.