You could say that Kayla Itsines has been in training for her current title, Instagram’s biggest fitness star, almost since birth. When she was a child and wanted to go anywhere — a friend’s, the playground, the beach — her father would tell her to do, say, 10 push-ups or 40 situps first.

Ms. Itsines, a 24-year-old personal trainer from Adelaide, Australia, doesn’t think this is odd. She tells this story happily, repeatedly calling her dad — a teacher and fitness enthusiast — “the best.” She also wasn’t allowed an Xbox or any kind of gaming console, she added, because both her parents wanted her to be outside, running around.

“I got it in my head to be fit when I was younger,” said Ms. Itsines, who played basketball from an early age. “I don’t have a sob story. I’m fit, I grew up in an awesome family in a beautiful place.”

Sitting in the W hotel while on a New York visit to surprise some followers, Ms. Itsines spent a full seven minutes enthusing about her boyfriend and business partner, Tobias Pearce, whom she met about three years ago at the gym (with help from Facebook). He accompanies her on all her trips.

Ms. Itsines’s life sounds idyllic, but unlike most people on social media, she shows very little of it. Instead, her Instagram feed, which has at least 4.6 million followers, is full of motivational sayings, silly e-card type sentiments (“I wish I could text my pets”) and photos of her abs, the definition of her six-pack so sharp you could cut yourself on it.

Most of all, there are often jaw-dropping transformations of followers who do her 28-minute high-intensity workouts, first released to the world in 2014 as the Bikini Body Guide (better known on social media as #bbg), an e-book that costs about $50.

Her sole vices appear to be taking pictures of her two Siberian huskies and cleaning — that Dyson vacuum cleaner post, she points out, was not sponsored. And yet women, particularly those younger than 30, can’t get enough of her. (Fans include the actress Allison Williams and the model Candice Swanepoel.)

There are nearly 1.4 million posts on Instagram using #kaylaitsines. Then there’s #thekaylamovement (1,047,385), #kaylasarmy (1,070,542), #deathbykayla (413,861), #bbg (2,814,016), #bbggirls (1,002,222) and more than a half-dozen others.

Women post progress photos of themselves in a bikini or underwear (Ms. Itsines does not believe in the scale, and it’s rare to see a “pounds lost” figure among her followers). They also post photos of license plates that read “I <3 BBG” and gel manicures that show fingernails painted with #BBG and a girl with a high ponytail (Ms. Itsines’s signature hairstyle) doing a squat.

The Instagram account names often have some permutation of bbg: erica_bbg, lizbet_bbg, Danielle.bbg, ruby_does_bbg. When Ms. Itsines appeared at free boot camps on a world tour last summer (more than 2,000 women showed up in New York), participants shrieked and cried — there are videos of this — at decibel levels rivaling that of a boy-band concert.

Women are so obsessed with her that they scroll all the way back to her first post, on Aug. 28, 2012, and comment breathlessly, as if they’ve finished a marathon: “MADE IT TO THE FIRST PIC! Over 2000 pics down,” and “Literally took me like 10 minutes to get here.”

Alex Siegel, 24, of White Plains stumbled on Ms. Itsines during a slow day at work in 2014. She has since fully committed, going so far as to give up alcohol (Ms. Itsines does not drink and calls alcohol “poison”) while working as a social media coordinator for a liquor company.

“You follow her for one day, and you want to be the person she features next who has had amazing progress,” Ms. Siegel said. Of Ms. Itsines’s personal appeal, she said: “I feel like she’s just really laid-back. It’s not like she’s going to all these celebrity spots. She’s not Kylie Jenner, here’s my nails, here’s my car, here’s my lipstick. It’s all very genuine.”

But, Ms. Itsines’s seemingly genuine could-be-anygirl image is tightly controlled. Between sales of her guides, her foam rollers, her water bottles and her recently released app, she’s almost certainly much wealthier than your average 24-year-old (or your average anyone), but she won’t confirm even how many employees her company has besides she and Mr. Pearce. (It was Mr. Pearce who encouraged her to create her guides after watching the response on social media to her photos of her clients’ transformations.)

There are at least two publicists, one and sometimes both of whom sit in on interviews, which may or may not explain why Ms. Itsines couldn’t think of an answer to a question about the biggest misconception about her.

“I don’t say anything I’m not meant to, or come across any way that I’m not supposed to,” she said, sounding sweet as raspberry tea, which is what she sometimes drinks when she craves sugar.

From her Instagram account, it’s nearly impossible to tell what the inside of Ms. Itsines’s house looks like. Followers never see her car, and a photo of her meals is very rare. Instead, she often posts beautiful photos, not necessarily even taken by her, of cut-up fruit, carefully avoiding species not universally available.

“If I post passion fruit, papaw, mango, and I’m at the beach in the middle of Australian summer and you’re in New York in winter, you look at it and you’re like: ‘I don’t have that. I feel unmotivated. I’m not following her anymore,’” said Ms. Itsines, who dropped out of college after a year to become a certified personal trainer.

Her ab photos — arguably the only thing she posts that could inspire an inferiority complex — rarely show her face, just a torso on a bed or the floor.

“I’m not like, you know, selfie!” Ms. Itsines said, making a duck face and pretending to take a picture. “If you feel good about yourself, you don’t mind helping other people feel good about themselves. I feel so good about myself that I don’t need to post me, me, me, me, me.”

As she spoke, her voice rose and she leaned forward, pressing her hands down on the sofa as if she was about to leap up with enthusiasm. She was wearing a rose-gold Apple Watch and white Nikes with white socks, the whites so bright they were almost fluorescent, and yet Ms. Itsines pointed out an invisible speck of dirt on the tongue of her right shoe that nearly prevented her from wearing it.

It would seem there isn’t much to object to in her feed, but of course, haters gonna hate.

First, there is the name Bikini Body Guide, which critics say suggests there is a single type of body — hers, perhaps — that can wear a bikini. (After explaining to countless reporters that for her, “bikini body” is merely a synonym for “confidence,” Ms. Itsines gave up and called her app the unarguable “Sweat With Kayla.”)

Second, last year Ms. Itsines sued an Australian raw vegan diet guru who calls herself Freelee the Banana Girl for claims, posted on YouTube, that included that Ms. Itsines was starving her followers with the 1,400- to 1,600-calorie diet she prescribes. The videos had spread to a war in comments on both Ms. Itsines’s account and those of her followers.

Ms. Itsines said she didn’t care what was being said about her — “As you can tell, I don’t have a sob story” — only how it affected her followers.

“We don’t want women feeling unmotivated,” she said. “Girls came crying saying: ‘Look, we feel really, really upset that someone said this about us. We’re not famous at all. We can’t take this.’” In a settlement, Banana Girl had to apologize and take down the videos.

Ms. Itsines has too much work to do to dwell on this. “But my work is like fun work,” she quickly added. “It’s not like I’m, ugh, on a laptop. I get to, like, travel around, take photos, make workout videos, I walk my dogs.” When she is not working, which she says is almost never, she and Mr. Pearce hang out with her family. “We watch TV I guess sometimes,” she said, after a pause.

Late at night, work done for the day, Ms. Itsines and Mr. Pearce squeezed in a trip to Rockefeller Center, just another young couple doing tourist things, except accompanied by two P.R. people. When they went to take a group photo, the lights went out, ruining the shot. There was nary a whiff of this disappointment on Instagram. Instead, Ms. Itsines posted a photo of her abs, looking even more chiseled than usual. After all, she doesn’t have a sob story.