When Ronda Rousey is training for a fight, she spends a week eating nothing but salty food. She wants to get bloated, so that when she eliminates salt from her diet, in the final days, her body expels all the fluid it can find. After a couple of steam baths, what remains of her weighs almost exactly a hundred and thirty-five pounds, the limit for the women’s bantamweight division of the Ultimate Fighting Championship. In the sport once known as cage fighting and now known as mixed martial arts, the U.F.C. is the dominant company, and she has become its dominant personality, despite the fact that not long ago its president was promising never to promote a fight between women. Rousey is a former judo champion, and she won her first eight M.M.A. fights with a move known in judo as juji gatame, which can be painful to contemplate, let alone receive: it is a type of arm bar designed to hyperextend an opponent’s elbow, stretching ligaments, tearing the articular capsule, and even grinding away the bone if the opponent doesn’t concede quickly enough. Outside the cage, Rousey is genial but unapologetic about her capacity to inflict harm. When, recently, she submitted to a brief interview on “American Idol,” Ryan Seacrest jokingly flinched as she greeted him. “I don’t fight for free,” she said. “Don’t worry.”

Although the U.F.C. packs arenas all over the world, it still isn’t quite mainstream, which means it is only an occasional presence on “SportsCenter” or sports radio. Its most important chroniclers can be found at the monomaniacal Web sites that keep track of its proliferating story lines: upsets and comebacks, crackups and busts, idle threats and infelicitous tweets. In this small world, Rousey’s ascent hasn’t been uniformly celebrated. In 2011, the editors of an irreverent blog called CagePotato declared Rousey their “new obsession.” These days, she is so polarizing that they can joke about “the M.M.A. commentsphere’s seething hatred of all things Ronda Rousey.” For her part, Rousey says she isn’t bothered by the evidence, online and in arenas, that many of the people who pay to watch her fight are hoping to see her lose. “I’m the heel, I’m the antihero,” she says. “And I like it that way.”

This February in Venice, California, it was still chilly when Rousey woke up, at four-fifteen, and made herself some eggs: spinach, turkey bacon, lots of pink Himalayan salt. She lives in a comfortably unkempt house, not far from the boardwalk, that she shares with three other female fighters—they call themselves the Four Horsewomen, in tribute to an old professional wrestling team—and a ninety-pound Argentinian hunting dog named Mochi. Her roommates were still asleep when she left to begin her daily commute: thirty miles across Los Angeles to the Glendale Fighting Club, a one-story anomaly on South Brand Boulevard, which is otherwise lined with luxury-car dealerships. She had to be there early for a series of live interviews with KTLA, which was interrupting its morning show to give viewers a preview of Rousey’s upcoming fight, her ninth, against a talented but relatively unknown wrestler named Sara McMann. The crew was setting up when Rousey walked in, wearing dark-blue stretch pants, a wide-neck teal top, and a black blazer—she had dressed in the dark, and she worried aloud that her outfit didn’t match. Her trainer, Edmond Tarverdyan, was more interested in talking about McMann. “You always look beautiful, and all that matters is you’re going to kick her fuckin’ ass,” he said.

Rousey smirked back. “Notice he didn’t say, ‘No, you match!’ ”

In M.M.A., more than in most sports, athletes must be promoters, too. Rousey is smart enough to know that one of her promotional assets is the way she looks—she has appeared on the cover of not only ESPN the Magazine but also Maxim, which called her “Badass & Blonde,” and photographed her in a garment that seemed highly unsuitable for combat. Of course, this asset can be a liability, too, especially for a female fighter seeking the same respect given her male counterparts. Rousey is five feet six, and even someone who didn’t recognize her might guess, glancing at her powerful arms and shoulders, that she was some sort of athlete. But while some fighters strike an impassive pose, shrugging off questions the way they shrug off the dangers of the cage, Rousey is nothing if not expressive. She smiles often, squinting so tightly that her eyes disappear. She cries easily, a girlhood habit she never outgrew. And before each fight she glares at her opponent as if she were getting ready to put a permanent end to a lifelong feud. After the fight, she is all smiles again, and usually unblemished. “Somebody told me once that it’s the pretty fighters you have to watch out for,” she says, slyly. “If someone’s all gnarled and mangled up, obviously they’ve been getting hit a lot.”

Rousey speaks more or less the way she fights: in measured provocations, never committing herself to a gambit that she can’t defend. When KTLA cut to her in the gym, she talked politely about McMann’s wrestling achievements, and about their parallel careers: McMann won a silver medal in wrestling at the Athens Olympics, while Rousey took bronze in judo at Beijing. The goal, after all, was to persuade fans to pay $54.99 to watch the two women fight, live from Las Vegas, on pay-per-view. But once the cameras left she assessed her chances more candidly. She predicted that McMann would fall back on her old wrestling moves for fear of Rousey’s brutal arm bar. “I don’t think that this matches up well for her,” she said. “I wouldn’t say that in a pre-fight interview, and I haven’t. Because it doesn’t make sense in order to sell it. I need people to doubt me.” She laughed. “And, besides, these guys”—she nodded toward Tarverdyan and his assistants—“put large sums of money on me winning, and they always get shitty odds. So I want to help them out.” She looked up. “Edmond, do you know the Vegas odds for this fight?”

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“Three-ninety-five,” he called back. A bettor would have had to lay three hundred and ninety-five dollars on Rousey in order to make a hundred if she won. Still, that meant the oddsmakers were giving McMann a better chance than they had given many of Rousey’s previous opponents.

Perhaps the observers in Las Vegas were impressed by McMann’s wrestling pedigree, or perhaps they were taking note of the events of 2013, a year when Rousey’s growing celebrity interfered with her training schedule. She took a ten-month break between fights, during which she acted in a pair of film sequels: “Fast & Furious 7,” which is due out next year, and “The Expendables 3,” with Sylvester Stallone leading a team of VHS-era action heroes (Arnold Schwarzenegger, Wesley Snipes, Dolph Lundgren). Rousey also served as a head coach on the eighteenth season of “The Ultimate Fighter,” the U.F.C.’s reality show, in which up-and-coming fighters live together while competing in a tournament to win a U.F.C. contract. She loved her team—two of its members are now her roommates—and hated everything else, especially the rival coach, Miesha Tate, whom she considers a phony. Rousey had fought Tate and beat her, two years earlier, in a short, devastating bout that did more than any other to make Rousey a star. When they fought again, last December, after the show’s conclusion, Rousey didn’t look quite so sharp: for the first time, she allowed a fight to progress beyond the first round. She was into the third when she finally isolated Tate’s left arm and bent it backward, at which point Tate used her free hand to tap lightly on Rousey’s leg, signalling submission and ending the fight. As they stood up, Tate offered Rousey a handshake and Rousey refused. “A handshake means something to me, and she hasn’t earned it,” Rousey explained in a post-fight interview. The arena booed her, confirming her paradoxical status as a popular heel.