In 2018 Olivia Palermo hired a social media agency to help build her online following. She already had five million followers on Instagram. That agency noticed something it thought was curious about her appeal.

“When we would show stuff of her private life, it wouldn’t perform as well,” said Jeffrey Tousey, the founder of Beekman Social.

This broke most of the understood rules of social media. You’re supposed to flaunt your glamorous life, but also, show your authentic self — here’s me putting on makeup before the gala, here’s me straightening my hair in a random bathroom.

It even extended to the old crowd pleaser: pets. “We’d post a picture of Mr. Butler,” Mr. Tousey said, referring to her dog, “and it wouldn’t perform.”

There was only one thing her followers (now 6.3 million) desired. “People want to see what she’s wearing,” Mr. Tousey said.

So Ms. Palermo is a famous personality who offers a very small window into her actual life, other than the occasional photo out with her German model husband, Johannes Huebl, or walking Mr. Butler, or, one taken this past summer, on a yacht in Greece, while vacationing with the designer Valentino Garavani.

You will never see her at home on a Sunday in sweats and a greasy ponytail. She probably does not do this off-camera either: Ms. Palermo wrote in a Q. and A. on the website of her trainer, Tracy Anderson, that “Sunday is not an excuse for looking badly dressed.”

Ms. Palermo, 33, first came to public attention a decade ago as something of a villain on the reality TV show “The City,” the fashion-obsessed New York City-based spinoff of the recently rebooted reality blockbuster “The Hills.” Her eye rolls became memes, and she was once featured in a roundup of “the 7 most hated people in fashion.”

But, unlike the majority of well-known people with a reality show pedigree, she is instead following in the tradition of coifed high-society swans of the past.

Her job is to be famously fashionable, in a series of roles that have ranged from model to chief executive. Her business, at first, was to serve as a tastemaker in the spaces that bridge runway shows to street-style Instagram accounts to Pinterest inspiration boards.

The influence of her polished, high-low style grew. She can wear a tailored blazer like nobody’s business, and pair a Zara skirt with a Dior top in a way that makes the mass piece look luxury and the luxury look approachable.

“Chic, but also with a certain nonchalance,” is how Mr. Garavani described Ms. Palermo’s style in an email. “Everybody should wear clothes in such a way that they never look straight off the runway.”

She began to work as a stylist, model or brand ambassador for Banana Republic, Tommy Hilfiger, Nordstrom, Ciaté, Rochas and Piaget.

In the fall of 2018, Ms. Palermo approached representatives of Karl Lagerfeld about collaborating. Six weeks later, she was in Paris meeting with the design team for his own label. In June 2019 — four months after the designer’s death, as it happened — the Karl x Olivia Palermo collection appeared.

Then she took greater charge of her own influence. She turned her longtime website into an e-commerce platform. She merchandises products through social media; she sells accessories for brands.

At last, she has founded her own fashion label: The Olivia Palermo Collection will appear in February.

Ms. Palermo is the model, the retailer and the design house all in one.

“She is writing a new category in the fashion world that didn’t exist,” said Giambattista Valli, the Italian designer of haute couture. “Before, there were people like Lee Radziwill. But Olivia is something new.”

Those society women of an earlier era — like Ms. Radziwill, Slim Keith and Babe Paley — didn’t have the technology, or perhaps the desire, to monetize a talent for dressing in such an all-encompassing way. Gloria Vanderbilt had her jeans empire, but she didn’t have shoppable Instagram.

“It’s not something that I woke up one day and said, ‘This is what I’m going to do,’” Ms. Palermo said. “I spent a long time on the process of building. I always have a clear direction. I am never one to not know what I am doing.”

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She first gained notice in 2006, as a pretty 20-year-old running around Manhattan in borrowed frocks with herfriend Byrdie Bell, attending every white-envelope charity gala and Zac Posen runway show, at least when she wasn’t going to classes at the New School.

It was a time when socialites like Tinsley Mortimer and Vogue editors like Plum Sykes and Meredith Melling were the “it” girls of the party pages (back when there were party pages).

Ms. Palermo was the youngest of that social set.

But now, Ms. Mortimer is a sometime-Bravo Housewife with a false lash line, and a generation of Vogue editors have all but disappeared, while Ms. Palermo is a chairwoman of the American Ballet Theater’s fall gala, goes on market appointments in Milan and Paris and vacations with Valentino. A recent afternoon found Ms. Palermo meeting with her team (the Olivia Palermo Group has eight employees) in her TriBeCa design studio, to select fabrics for her fashion line and choose the exact right shade of green for the branding.

“Of course, it was fun,” Ms. Palermo said of her party days. “But within all of that, it was important to figure out what I wanted to do, other than to run around and have a good time.”

She also ran the risk of being permanently identified as a mean girl. From the first episode of “The City,” when Whitney Port met Ms. Palermo in the offices of Diane von Furstenberg — they were both interns — it’s clear Ms. Palermo’s character is the fashion mean girl with the entitled attitude.

“We don’t go out to cast a villain,” said Adam DiVello, the show’s creator. But, “I think in Olivia’s eyes, she came across as the not-so-nice one. I used to say, ‘Olivia, people just love you on the show.’”

And her positioning as the snooty posh one is more complicated. She did grow up in Greenwich, Conn., and on the Upper East Side, and attended private schools.

But she lacks a storied family name or great fortune. Her father, a real estate developer, has been used as tabloid fodder for his legal and business troubles, including a bankruptcy filing. Her mother is an interior designer, a career that’s often more wealth serving than wealth making. Her parents divorced when Ms. Palermo and her younger brother, Grant, were children.

She has done very little TV since “The City.” Many celebrities achieve early fame by things one may find embarrassing — the Mickey Mouse Club, sex tapes, marrying Tommy Mottola. Reality TV is the embarrassing lark that made Ms. Palermo famous, but that now dare not be mentioned.

Ms. Palermo did carefully express irritation at how she was portrayed, despite the name building that came from it.

“Whoever sat in that editing room was very talented at creating a human that I was the polar opposite of,” she said.

But all reality shows are at least a little phony, aren’t they? Ms. Palermo couldn’t help but raise her voice.

“No, no, no,” she said. “It was fully phony. There was nothing real about that show at all. Other than our first and last names. Nothing. Absolutely zero.”

The real Olivia is so nice that she brings cookies to the fashion publicists who have lent her clothes. She is so nice that she exclaims “Goody gumdrops!” on hearing happy news. Far from being a fierce fashionista, Ms. Palermo can be a little dorky. Her favorite pastime is playing Mario Kart.

But the Olivia on “The City” was compelling. During two lunches, several hours and an evening out with her now, for reporting for this article, it was hard to get any sense of her other than “nice.”

Her niceness seemed in some ways like a shield against personal disclosure. She could seem highly controlled in conversation, especially when discussing her family history, and spoke in generalities, even about her favorite subject, fashion.

At other times, she seeded her statements with the equivalent of paid content. When asked for her favorite moments from the fall shows in Europe, Ms. Palermo cited two labels (Valentino and Giambattista Valli) to which she had personal connections, and a third (Max Mara) with whom she has a business relationship.

As a concept and a public person, she remains a surface.

Her Instagram posts have the composed beauty of a magazine image, not the candid quality of, say, Tavi Gevinson being silly on Instagram Stories. With such distance, Ms. Palermo can’t truly be an influencer, and besides, her fame predates the rise of social media.

Her outlook is “old-world, old-school,” as she put it. She is so restrained that some conservative religious women have, for years now, seen her as a style model and a paragon of modest dressing. In the age of oversharing, she describes herself as “very private.”

What do we know?

Edward Barsamian, the head of editorial at Victoria Beckham and a former Vogue.com editor, met Ms. Palermo when they were teenage interns at Quest, an uptown lifestyle magazine.

“Her work ethic and drive bonded us,” he said. “Whether it was a C.Z. Guest event or going on a J. McLaughlin market appointment, she put her nose to the ground and did the work.”

Karen Diamond, the director of Models 1, an agency in London that has represented Ms. Palermo and her husband, Mr. Huebl, for many years, said she has an encyclopedic recall of designer collections. She also has a fiery side and a naughty humor.

“She likes risqué jokes,” Ms. Diamond said, “which is at odds with the public image.”

And what about her personal life?

In a rare moment of openness, Ms. Palermo relayed the story of meeting her husband at a movie screening when she was a 21-year-old student. On their first date, “I was in Converse sneakers and jeans and had a golf umbrella,” she said. “We’ve been together ever since.”

Maybe Ms. Palermo has adopted a guarded public persona, having felt burned by her reality TV experience and her brushes with tabloid viciousness. Perhaps she wants to protect her family and her private life.

It’s also possible that it’s all just business. Maybe she understands better than anyone the business imperative of what her fans want from her: fashion, not personality.