If you want to understand why Kevin Sharkey is Martha Stewart’s most trusted employee and best friend, consider how he treats his less famous companions: Once, for a cross-country drive that involved overnight stays in dingy motels, Mr. Sharkey brought along pillows, perfectly pressed D. Porthault linens and bolts of painter’s tarp.

Upon arriving at the first motel, Mr. Sharkey “rolled two enormous T. Anthony suitcases out of this 30-foot Penske truck and started wrapping mattresses in plastic tarps and making the beds,” said the photographer Douglas Friedman, the road trip’s raison d’être. He had asked some friends, including Mr. Sharkey, to help him move from New York to his new home in Marfa, Tex. “That’s Kevin,” Mr. Friedman said. “He also brought clean sheet sets for the next nights.”

It’s all part of the philosophy he’s internalized over 23 years of working with the world’s most famous lifestyle guru: Things don’t have to be perfect. They just have to be close.

Officially, Mr. Sharkey is the executive vice president and executive design director of Martha Stewart Brand Management for Sequential Brands Group. Unofficially, he is her deputy in all things.

Of her 94 books, she has a co-author on just one: last year’s “Martha’s Flowers,” written with Mr. Sharkey. He is the only person other than Ms. Stewart to have been featured on the cover of Martha Stewart Living. He is typically the last person before her to approve a Martha Stewart project. And he has an apartment or room at all of her homes.

“He’s her consigliere,” said Pilar Guzmán, the former editor in chief of Martha Stewart Living magazine and Condé Nast Travel, who is now a writer and media brand consultant. “He’s the voice of reason whispering in her ear.”

Mr. Sharkey, 50, joined the media and merchandising lifestyle empire that is Martha Stewart in 1996, after working for several years as the assistant to the renowned decorator Albert Hadley at Parish-Hadley Associates, a design firm whose old-guard clientele included the philanthropist Brooke Astor.

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Mr. Sharkey goes flower shopping more than once a week.CreditNathan Bajar for The New York Times

At Martha Stewart Living, Mr. Sharkey took lunch orders, filled out FedEx forms and packed props before eventually becoming the decorating editor. Now, he oversees a team of 20 designers, most of whom dream up or approve products and ideas for the 40-some companies that encompass the Martha Stewart brand, which was purchased in 2015 by Sequential Brands Group. (That company now does licensing for the thousands of Martha Stewart items made by or in partnership with companies like Amazon, Macy’s, QVC, Staples and Pat LaFrieda meat.)

His name is familiar to readers of Ms. Stewart’s blog, and he has appeared often on her TV programs. (He also hosted a humorous radio show until 2015 on Sirius XM on gardening and design.) But few know how indispensable Mr. Sharkey is to the ultimate household name in all things household.

Mr. Sharkey was raised in Boston, one of two boys in what he called “a traditional family.” He went to an all-boys Jesuit high school, and at home, there were Great Danes and holidays marked by extensive decorating. The big influences on his young life were his childhood visits to the Isabella Stewart Gardner Museum, his summer job at Harvard’s Arnold Arboretum and the Rhode Island School of Design, where he majored in architecture and established his pack of friends, many of whose children he godparents.

One of those friends, the decorator Alexa Hampton, who typed up Mr. Sharkey’s first résumé on the floor of her dorm room at Brown University, knew he was destined for a career in design: “His work has always been a calling for him, not a vocation.”

Good taste came naturally to Mr. Sharkey, but he needed to learn the rigors and subtleties of running a business. Both of his mentors taught him that, away from the glare of the spotlight.

“It was fun for me to be standing behind Mr. Hadley just like it is fun for me to be standing behind Martha,” Mr. Sharkey said recently. “People reveal so much. They aren’t revealing it to you. You’re just observing it.”

He described Ms. Stewart as both a role model and a student of life. “She’s a teacher and she’s driven to learn,” he said.

But he wanted to make one thing clear: “I am not Martha Stewart’s pet. I work, and I work hard. And I love what I do,” he said. “I would be disappointed if somebody didn’t understand that.”

He continued: “I can tie a ribbon on a wreath with no wreath and no ribbon. I can make anything look good.”

Trey Laird, a fashion and luxury brand consultant who is a friend and colleague to both Ms. Stewart and Mr. Sharkey, said they have “a mutual respect for each other’s creativity and skills.” Mr. Laird said: “I think he holds the brand inside him. He gets it and he can apply it seamlessly.”

And he has a special power over Ms. Stewart.

“I’ve seen Martha get prickly as many people can, and he has this wonderful ability to diffuse the situation,” the interior designer David Kleinberg said of Mr. Sharkey. “He’s kind of like Martha’s Xanax.”

There’s only one thing he can’t do.

“I don’t cook,” Mr. Sharkey said, standing in the gleaming white kitchen of his New York City apartment on Charles Street, which overlooks the Hudson River through floor-to-ceiling windows.

CreditNathan Bajar for The New York Times CreditNathan Bajar for The New York Times

Mr. Sharkey’s home, in a building designed by Richard Meier, reflects his understated taste and meticulous organization. A long corridor is lined with a color series by the artist Josef Albers, a gift from Ms. Stewart. The wall of his library is taken up by a photograph depicting tall firs and pine needle paths that lead to ponds at Ms. Stewart’s home in Maine. (She had it enlarged for him after he admired it on a photo shoot.) There are Blanc de Chine porcelain figures and wooden Guanyin sculptures, and low sofas and tables that allow for uninterrupted views of the Hudson.

He is also renovating: Soon, there will be new woodwork — cupboards in the dining room and bookcases in the bedroom — from the California Closet company, a partner of Martha Stewart, and a new paint color to replace the soft khaki (called “Heath,” a colorway from an old Martha Stewart paint line) that currently covers all of the walls, floors and ceiling. (It’s been a topic of heated discussion for Mr. Sharkey and Ms. Stewart. “You saw the stripes of paint he is testing in his apartment? I hated all of them,” she said, “because they just aren’t the right color gray.”)

Mr. Sharkey entertains here; on any given night, friends cook or he orders pizza or Chinese food. The dining table becomes a table-tennis table. On Sundays, guests gather to watch movies in his building’s screening room. Everyone gets candy.

“I don’t have a partner or a boyfriend,” he said when asked, and joked that press coverage could result in “marrying me off.” Work and socializing tend to run together for him, but, Mr. Sharkey said, he is most proud of his enduring friendships.

“He has all these parties and I’m not included,” Ms. Stewart said jokingly from her Manhattan office, Mr. Sharkey sitting close by her side.

“I have cocktail parties and I invite you!” Mr. Sharkey said in protest, a flush of pink coloring his cheeks.

“I’m jealous of all of his other friends,” Ms. Stewart continued, with a twinkle in her eye. “They go off on their trips and he doesn’t invite me.” She listed the names of people with whom Mr. Sharkey takes an annual trip to Italy on July 4 — the only holiday they spend apart — while he sighed audibly and widened his eyes in showy yet amused exasperation.

CreditNathan Bajar for The New York Times

The two vacation often and adventurously together, sometimes with Ms. Stewart’s two grandchildren and her daughter, Alexis. There are safaris in Botswana and Zimbabwe, visits to the home of the designer Valentino Garavani, and private tours of the gardens at Tokyo’s Imperial Palace and Prince Charles’s Highgrove House. Mr. Sharkey also hikes with Ms. Stewart when her other friends refuse.

“It’s very serious, his effect on my state of mind, in terms of personal things,” said Ms. Stewart, who describes Mr. Sharkey as her best friend and someone “who is not obsequious in any way, just fun and intelligent.” And her devotion to him is manifested in tributes. Ms. Stewart has named a French bulldog after him, as well as a leather tote, a shade of paint (“Sharkey Grey” was exclusively sold at Home Depot) and a cartoon character in a stiffly mannered children’s animated web series she created.

They share a sense of humor, by turns deadpan and goofy. (See: A 2013 TV segment in which they taste test a table full of hot dogs sourced from the tristate area, giggling over sly innuendos.) And the two have a zeal for immaculate spaces. Ms. Stewart’s eyes lit up when she recounted calling Mr. Sharkey on an ordinary Sunday to ask what he was doing only to hear “cleaning, organizing a closet, ironing.”

Alexis’s children call Mr. Sharkey “Tio Kevin,” and pop in his apartment when they wish. He takes Jude to her dance classes and lunch on Saturdays and has a weekly “gentlemen’s dinner” with Truman, a soccer fanatic who is allowed to practice kicks at home and at Mr. Sharkey’s apartment, which is just below Alexis’s.

“It’s like his house, and he can just walk in,” Alexis said. “He’s such a close friend of mine, I wouldn’t have it any other way.

More than once a week, Mr. Sharkey peruses the outdoor flower market in Manhattan’s Chelsea neighborhood, looking for inspiration and gifts — because it’s always someone’s birthday. Mr. Sharkey has many other old-world manners, offering his arm to a woman on the stairs and writing thank you cards on good stock even after a casual supper.

But his sense of humor tempers his Boston Brahmin accent, herringbone tweed coats with fur shawl collars, Thom Browne suits and tapered-toe leather shoes. So does his fondness for Fritos paired with caviar. There’s a photo that Ms. Stewart once showed on TV of him asleep in the bow net of a Chinese junk ship, clutching a beer bottle.

“He is always very correct, very proper, but somehow remains a fun, little kid, who should be playing Puck in ‘A Midsummer Night’s Dream,’” Mr. Kleinberg, the designer, said.

The inseparable duo at Martha Stewart headquarters in Manhattan.CreditNathan Bajar for The New York Times

Mr. Sharkey’s office at Martha Stewart’s headquarters in Manhattan is technically windowless, but he says he has a “million views.” Hundreds of books are arrayed, floor to ceiling by color, and dozens of Martha Stewart employees work quietly nearby. The walls are pinned with images of sunlit gardens, sleek furnishings and salted caramel cookies.

When she was freelancing as a stylist for Weddings, Wendy Goodman, a writer who specializes in interior design, said Mr. Sharkey’s grand floral displays left her “breathless.” His talent, self-effacement and respect for colleagues are an “intoxicating combination,” she said. “You just want to be with him every day.”

Darcy Miller, the former editor of Martha Stewart Weddings magazine, spoke of him as an office confidant and a candid colleague. “He can say things to her no one else can,” she said.

For decades, Ms. Stewart endured snarky profiles about her craft projects and sendups of her perfectionism on shows like “Saturday Night Live.” These days, as the spare, natural-lit and handmade style she championed has been widely imitated — by Kinfolk magazine, Pottery Barn and others — Ms. Stewart appears to have the last laugh. She even owns and monetizes her caricatures.

“Martha & Snoop’s Potluck Dinner Party,” an Emmy-nominated cooking show on VH1 with the rapper Snoop Dogg that has Ms. Stewart downing tequila shots, was renewed for a third season. And her surprisingly blue roast of Justin Bieber for Comedy Central, where she joked about surviving the five months she spent in prison beginning in 2004 after being convicted of lying to investigators about a stock trade, was a hit. At 77, Ms. Stewart — still a competitive business woman of “protean competence,” as Joan Didion once wrote in The New Yorker — walks a little slower, albeit in five-inch Prada booties. But she’s still expanding her empire, and also, has probably plowed four miles of snow from her farm in Bedford, N.Y., before you’ve switched on the coffee maker.

In March, she announced she would advise a Canadian cannabis grower on hemp-based CBD for people and pets. And she has found other platforms for her merchandise: Last February, she missed her beloved chow chow Empress Qin compete (unsuccessfully) for “best in show” at the Westminster Kennel Club Dog Show in February because she was selling her trademark puffer jacket on QVC.

Amid it all, Ms. Stewart has not chosen a successor. Asked if she envisioned handing the reins to Mr. Sharkey, she smiled, arched a brow and said, “You never know.” She mused aloud about whether Coco Chanel retired or worked until her death — Ms. Chanel died in her 90s in a middle of designing a collection — and said, lightly, “maybe I’ll have to die.”

“I hope I’m picked,” Mr. Sharkey said in response. “It would be so sad if I had to kill someone else because they got that job. Because I would.”

Ms. Stewart giggled, looking pleased.