She never takes a day off, not even on Sunday. She works out of the Oval Office and she’s got a regular seat on Marine One. She texts with world leaders on the president’s behalf. She stays up late with him drafting social media posts.

She rarely speaks on camera and yet she is never not there, ever-present with a wide smile and her eyes fixed on the president.

She is Natalie Harp, the 35-year-old West Wing aide who is the utmost gatekeeper to President Trump, a one-woman conduit of information operating entirely outside the usual chain of command. The public’s interest in Ms. Harp exploded after she was revealed last week to be one of the lucky few who joined the president inside an airplane catering container to be whisked out of Turkey under threat of attack by Iran.

This week she became the subject of a political attack line delivered by Jon Ossoff, the Democratic senator from Georgia, who said of the president: “He doesn’t want to do the job. He wants to build his ballroom and travel with Natalie on their apparently defenseless flying palace gifted by the emir of Qatar.”

The senator’s singling out of Ms. Harp triggered a particularly ferocious response from many corners of Trump world. White House aides, lawmakers on Capitol Hill, conservative influencers and at least one cabinet member took to the ramparts on X to defend Ms. Harp’s honor. There were accusations of sexism from the right and cries of snowflakery from the left. Mr. Trump deflected when a reporter from CNN asked him about Mr. Ossoff’s comments. But a social media account run by the administration then attacked the reporter in unusually personal terms, invoking her children.

The furor intensified the spotlight on this seldom-heard-from and sylphlike woman who has long fascinated and frustrated people around the president. This article is based on interviews with a half-dozen people with knowledge of her role. Ms. Harp did not respond to a request for comment.

The unique power Mr. Trump affords her says much about how he functions as president.

As long as Ms. Harp is around, he enjoys an unvetted flow of information, some of it poisonous and counterproductive, at least as far as a number of his other advisers are concerned. She supplies him with content she fishes out of the social media gutter (like the video he posted depicting the Obamas as apes) and provides him with a view from the world outside official Washington, of which he has always been mistrustful. Her moniker is “the human printer,” because she tails the president with a portable printer so she can hand him information in hard copy.

She is a platinum blonde geyser of positivity, there to buck him up when the going gets rough. She believes, or at least vehemently claims to believe, that he totally won the 2020 election. In the book “Regime Change,” by the New York Times reporters Maggie Haberman and Jonathan Swan, Mr. Trump is described as explaining to people that Ms. Harp is the only one who loves him as much as his wife and children. All his other staff will go off and make money, Mr. Trump would say, but Ms. Harp will never leave him.

“Natalie Harp is one of the most loyal and hardest-working aides on President Trump’s team,” Karoline Leavitt, the White House press secretary who will soon be leaving her post, said in response to questions from The Times about the nature of Ms. Harp’s position and why she was chosen to join the president on the secret food-delivery-truck rescue gambit.

In some ways, Ms. Harp is filling a role that Mr. Trump has always required, going back to his days as a real estate developer in Manhattan. Back then, there were always secretaries sitting outside his office on the 26th floor of Trump Tower, ready at a moment’s notice to hop to it. He referred to these women as “the girls.”

Ms. Harp collects a taxpayer-funded salary of $150,000. Her double-barreled title is special assistant to the president and executive assistant to the president. At work she can be a bit of a loner. Even in this West Wing, in which loyalty to the president seems to know no bounds, Ms. Harp’s displays of devotion have unnerved some of her colleagues.

Several videos shared with The Times show Ms. Harp sprinting across great distances to keep up with Mr. Trump’s golf cart as it putters across his course in Scotland. “I also am sorry if I was an embarrassment walking the course in Scotland,” she wrote in a letter to the president that was viewed by The Times.

The letter goes on to say: “I want things to always be right between us. I also know I’ve been distracted all week (forgetting to eat throughout the days, and even forgetting to sleep, and only catching a couple hours at a time).”

The letter concludes: “With all my heart, Natalie.”

She grew up in California. On Wednesday, her brother, with whom she is apparently estranged, gave an interview to CNN. He said this about his sister: “I’d say since she was, like, 16, she’s had an obsession with, um, gosh, how do I explain this? Um, she’s written letters to other presidents, let me put it that way.” (He said he was referring to George W. Bush).

Ms. Harp’s connection to this president runs deep. She credits him with saving her life.

She has said that she suffered from bone cancer and that it was legislation Mr. Trump signed in 2018 that provided her access to experimental treatments. (She has not publicly detailed which treatments).

She was recounting that experience on Fox News in 2019 when she caught the president’s eye. He invited her to speak at the Republican National Convention in 2020. She spent the next two years repeating his election lies as an anchor on the far-right One America News Network. In 2022, she finally joined his team.

A “War Room”-style documentary filmed during Mr. Trump’s 2024 campaign, called “Art of the Surge,” provides glimpses of Ms. Harp hard at work.

“Politics was never something I liked,” she explains in one scene that was filmed at Mar-a-Lago. “But when I was sick, I started watching the Republican debates. He was my first exposure to politics, and I loved it. Like, this guy, he gets it.”

The film cuts to Mr. Trump introducing Ms. Harp in 2019 at something called the Faith & Freedom Coalition’s “Road to Majority” conference.

“They were preparing her for death,” he says. “Her name is Natalie Harp, and because of ‘right to try,’ she’s now living, and I think doing phenomenally well. And somebody said she’s here. Are you here, Natalie? Where’s Natalie? Will you come up here, please?”

The woman who walks across the stage to hug him is not yet the full-fledged Trump aide she would soon become.

“I had cancer and he saved my life, signed that bill,” she says in the film, “and ever since he found out, he stayed in touch with me. Made sure I was OK.”