Before an audio tape of President Javier Milei’s sister was recently leaked to the news media, most Argentines had never heard her voice.

Karina Milei, perhaps the second most powerful person in Argentina, had largely kept silent, pulling strings from behind the scenes while her boisterous brother commanded the stage.

But now Argentina’s mysterious first sister, who quietly helped fuel her brother’s rise from TV pundit to president and a leader of the world’s populist right, has become a focal point for a public test of his government and a lightning rod for corruption accusations leveled against it.

While the tape attributed to her was innocuous, rallying party members and telling them to stay united, a separate tape attributed to a different government official suggested she was profiting from a bribery scheme.

Competence is captivating. No need to tell viewers tuning in to Gov. Andrew M. Cuomo’s daily media briefings. Many have been riveted by his plain talk in the face of a global pandemic.

Others are increasingly focused on the woman usually seated at his left: the one who on a recent Thursday countered a reporter’s suggestion that New York had dragged its heels in response to the spread of the coronavirus by assuring him in a tone like flint that once it was clear that New York, and not just the West Coast, was an entry point for the virus, “we shut down fully, with literally zero guidance from the federal government.”

She is Melissa DeRosa, the trusted aide and strategist to whom Mr. Cuomo routinely turns for a reliable litany of figures and facts: often seen taking notes, checking her cellphone and sliding relevant documents under the nose of her boss. A steady presence on the dais, Ms. DeRosa, 37, is having an extended breakout moment.

As secretary to the governor, she is the most powerful appointed official in the state, the first woman to hold that position, and one of the youngest, her robust command of facts and arcana only hinting at her sway.