The young woman who approached the table by the window at the Pinkberry on Eighth Avenue and 18th street was nervous, breathless. She was sorry to interrupt, sorry to be a bother, she said. It was just that she loved Taylor Momsen so much.

The just-barely-15-year-old Ms. Momsen, who plays aspiring queen bee Jenny Humphrey on the CW show Gossip Girl, gave the woman—who looked to be at least in her mid-20s, and was clutching a napkin and shaking like a whippet in winter—a warm, practiced smile and helped her locate a pen and piece of paper so she could sign the autograph on something suitable. Ms. Momsen scribbled her name with a girlish curlicue flourish, and offered a friendly wave goodbye as her fan skipped elatedly out the door.

It’s a regular occurrence now for Ms. Momsen (“The older girls get nervous. The younger girls, cry,” she said), who, over the summer, has become the obsession of a strange and ragged slice of New York. Even as her co-stars have ascended the ladder of gossip-worthiness—the golden-locked goddess Blake Lively and the impressively sideburned Penn Badgley have had a very public romance while dandy-dressing, secretly English Ed Westwick has hardly been shy about his carousing—Ms. Momsen has emerged as the show’s true It Girl: admired, lusted after and, of course, scorned. Girls and young women covet her style (which ranges from bright, simple and sophisticated to mix-and-match punk). Boys and men—in their teens and 20s and beyond—can’t help but be drawn to her coltish beauty (even as that fact could make them uncomfortable). And there are the requisite haters, too, so unnerved by Ms. Momsen’s uncanny poise and sudden ubiquity that they can’t help but snipe at her. As photographs of her—on location for Gossip Girl, making the rounds at parties and movie premieres, riding the subway around her newly adopted city—have cropped up on celeb-tracking blogs and Web sites, they’ve also collected captions and comments snarky and vicious enough to be straight from the cell phone of Gossip Girl’s eponymous blogger.

“Our thanks to Taylor Momsen and Ali Lohan for making 14 the new 35,” wrote a commenter on Gawker, under a photo of the actress in a froofry yellow frock at the Fifi fragrance awards in May. Captions for other snaps have included “Little Bitch Stops for Itch” (this was later changed) and “Taylor Momsen Forgot her Pants.”

But is it Taylor we love to love, and love to hate? Or Jenny, who practically ran away with the show last season as she plotted her path to the top of the prep school food chain? Ms. Momsen understands how people might get confused.

“It’s funny because I forget that I look the same—you know, that I look the same all the time,” said Ms. Momsen, as she spooned original-flavored Pinkberry yogurt covered in Fruity Pebbles into her mouth. “I have two different modes: I have my on-set mode, and then I have my mode with my friends and family. My Taylor mode. And when I’m in my Taylor mode, I forget that people still see me as Jenny. It takes a minute.”

If we can’t remember, it may be because we don’t want to. She may be a teenage social-climbing bitch, but Jenny Humphrey has lifted Gossip Girl from sudsy teen drama to genuinely gripping television, and brought us the most lovable villain we’ve seen in ages.

The New Eve Harrington

A day before our Pinkberry date, at the Silvercup Studios set of her show, Ms. Momsen was working through a scene that would be familiar to any regular Gossip Girl viewer. It was breakfast time at the Humphrey loft, and artificial sunshine streamed through the fake windows, drenching the very-hard-to-believe-this-family-is-supposed-to-be-poor loft with light, while the Humphreys themselves bantered wittily around a kitchen counter. For those who haven’t fallen pray to the show’s charm, a primer: Gossip Girl peeks into the rarified air and lives of the Upper East Side’s privately schooled elite. Based on the best-selling series by Cecily von Ziegesar, and developed by O.C. masterminds Josh Schwartz and Stephanie Savage, the show is a delicious weekly Greek tragedy, filled with scandalous sex, betrayals, drinking, drugs, homosexuality, embezzlement, sinister plotting and social hierarchy coup d’état, all recorded on the anonymous Gossip Girl blog. Each gleaming setting—the shiny town cars, the opulent limestone townhouses, the bejeweled department stores—and reference to exclusive New York City hot spots screams privilege and invites a peek into that ever-lusted-after moneyed unknown (Ms. Savage has previously said that the Sofia Coppola film Marie Antoinette helped inspire the look of the show).

Jenny’s family isn’t a natural part of the socialite spectrum (they live not in the East 80s but in—gasp!—Brooklyn), and she and her older brother Dan (Mr. Badgley) are the perpetual outsiders. When we met them in the first season, Dan was a brooding aspiring writer (dubbed Lonely Boy by Gossip Girl) who found himself thrust into the inner circle by dating über-chic Serena Van der Woodsen (Ms. Lively), while Jenny was a wide-eyed naif, awed and intimidated by the rich and glamorous popular girls. Dan kept his head straight for the most part, but Little J couldn’t help but make a power play for the top, initiating a tug of war with Blair Waldorf (Leighton Meester), who may be the scariest raven-haired actress we’ve seen since Shannen Doherty stormed off 90210.

Over the course of season one’s 16 episodes, Jenny slowly but surely morphs into a girl exactly like those who initially shunned her. Her transformation involves, at various points, lying, scheming, stealing a $15,000 one-of-a-kind Valentino dress and dating a rich closeted gay boy in order to raise her social status. She’s downright literary: Think Edith Wharton (it can’t be a coincidence that one Gossip Girl character, “Lily,” is married to “Bart,” a wink to The House of Mirth’s main schemer, Lily Bart). Or Eve Harrington, that ingénue turncoat. Jenny is every bit as fascinating, attractive and appealingly repellent as these famous characters, with their insatiable ambition to conquer a world that is being silver-spoon-fed to their rivals.

“With Jenny, there’s that sense of a very intelligent, observant young girl looking around and going, ‘You know, I’m not as rich as these girls, and I don’t have that leg up because of my family, but I’m just as pretty and I think that I’m smarter, so if I play my cards right, I can win this game,” said executive producer Ms. Savage, who noted that at some point in the new season a character actually refers to Jenny Humphrey as “Eve Harrington.”

In the books, Jenny is a short, busty, curly-haired gal who is constantly getting trampled on. The creators decided early on to take the character on the show in a different direction, Ms. Savage said. “We wanted a character that in this journey of trying to fit in at school might actually succeed in her goal and even triumph over the other girls at some point,” she said.

The second season, which starts giddily and with a bang in the luscious setting of the Hamptons on Sept. 1, has aspiring fashion designer Jenny working as an intern for designer Eleanor Waldorf, who—of course!—is the mother of chief rival Blair (“Jenny has put high school to the side and focuses on her career, trying to be a fashion designer, and to fulfill her goals of world domination,” laughed Ms. Savage). In the scene filmed that afternoon at Silvercup, Jenny is reprimanded for ignoring her schoolwork by her father (Matthew Settle). The camera stayed on Ms. Momsen, clad in impossibly short denim shorts and a tank top, honeyed hair cut shorter and styled more edgily than last season, as she reacted to her lecture with teenage-appropriate indignation, eye rolls and exhales. After the second take, the director called out his praise, his only note to “sprinkle it with more attitude.” Ms. Momsen nodded, the cameras rolled, and she redid the scene: eyebrows imperceptibly more arched, eyes ablaze, disgust apparent and with an added angry toss of a cereal bowl spoon as she huffed off the take. Next to this Observer reporter, Collette Momsen, Taylor’s mother and on-set guardian—as youthful and beautiful as one of the parental characters on Gossip Girl, dressed in skinny jeans and very high heels—glanced at the monitor and laughed softly. “That’s a familiar look,” she said fondly.

Model Child

Asked where she’d like to meet for a late-afternoon meal, Ms. Momsen chose the Pinkberry near the Chelsea apartment she’s living in with her mom and younger sister, Sloane. She is tall and striking, and that day was dressed like other young teens out on the street—jean cutoffs, a white T-shirt that hung casually off a slender shoulder, dark nail polish on her short nails, sparkly pink Tarina Tarantino jewelry, and lots of eyeliner—but with that indefinable shimmering charisma that had fellow Pinkberry patrons of all ages doing a double take. Her legs are epic, long and stretching out from here to forever—and, she pointed out, covered with nicks and cuts.

“They take up most of my body, and I’m not used to coordinating them,” she said. (She did, in fact, later trip on the sidewalk, and she still seemed embarrassed when she told the story of her grand entrance her first day on the Gossip Girl set when she wiped out before she could even say her name.)

At certain moments, she has the full cheeks and pout of a young girl, but with a toss of her hair she morphs into a bona fide sophisticate. All this serves her well on Gossip Girl (and in film roles like the recent Gus Van Sant’s Paranoid Park): She can play the kittenish innocent as easily as she can own the vamp. And she does share one important quality with Jenny: She “love[s] high fashion.” Ms. Momsen recently signed as a model to IMG and shot a 10-page spread for the October issue of French fashion magazine Crash.

In conversation, too, Ms. Momsen seems to swing back and forth between the polished industry veteran she is (she started working before she could talk) and a newly 15-year-old girl. When talking about her hit show, for example, she is politic, professional and unfailingly gracious—careful not to leak any plot points, making sure to praise the cast and crew, talking (despite persistent rumors) of the feeling of harmony and family on set. But when conversation switched back to her friends, her favorite bands (Garbage, Paramore), her hopes to one day record an album of her own songwriting and music, to her dog, Petal—a 6-month-old white Maltese with black (“punk rock”) streaks on her head—she is transformed: girlish and animated and super-chatty. She said she mostly doesn’t remember her early acting days—the baby modeling, the national commercials she did as a toddler, her co-star Jim Carrey when she was cast as Cindy Lou Who in 2000’s How the Grinch Stole Christmas—or even really the fun stuff, like walking the red carpet, going on Jay Leno or meeting the queen. She said people are always asking if she chose this life for herself. “It kind of chose me,” she said. “I had the opportunity to say no, but I love it. I’ve grown up in it. It’s kind of all I know.”

As the only cast member under the age of 18 (and probably one of the only actors who is actually the same age as the character she plays), she is the lucky one who gets to leave the set after 9.5 hours. The Momsen family moved to Potomac, Md., for her father Michael’s work, when she was 11, and they currently shuttle back and forth while Ms. Momsen lives and works in the city. Her best friend, Sammy, still lives in Maryland, and the two girls visit each other often. Her New York friends are mostly fellow students at the Professional Performing Arts High School, where she will be a sophomore if she doesn’t opt for home-schooling this year. (Connor Paolo, who plays Eric Van Der Woodsen on Gossip Girl, recently graduated from there and was the one who convinced her to attend when she was shooting the pilot.) For her birthday in late July, a large group of her friends all dressed up to attend a midnight screening of The Rocky Horror Picture Show. Ms. Momsen wore a pink tutu—it’s been a birthday tradition for her since she was 3—with shorts underneath, a sequined corset top (“it sounds really bad, but it was really cute”), and a dark metal-and-pearl tiara and “fierce” 6-inch high heels. “I had the worst blisters the next day,” she said happily. She shrugged off the glare of being in the public eye. “You can always go into an apartment,” she said.

“When Josh and I met Taylor,” said Ms. Savage, “she really had that great quality; on the one hand being Cindy Lou Who, really sweet and just sort of delightful and smiley and funny, and she can act her age. … But she also has that Gus Van Sant side to her—very grown-up, very self-aware. There is an intelligent, watchful darkness that she’s got as well. Those two qualities flickering back and forth between the sweet and the innocent and the more intelligent observer felt like that was really something that could be something magical for the role.”

Clearly, Ms. Momsen’s magic has worked on New York. As she left Pinkberry, a group of whispering teenage girls tried (and failed) to inconspicuously trail Ms. Momsen. After about five blocks, they worked up the courage to ask for a group photo. Turning on her Taylor smile—which yes, looks just like Jenny’s—Ms. Momsen complied, happily.