MK, as she is known to her friends and family, is also a punctual and professional sort. She arrives for a poolside tea in Los Angeles 10 minutes early, ordering a hot chocolate while explaining her fetish for all things sweet — "I'm a candy girl, like Tootsie Rolls and Swedish Fish" — and objecting when the waiter tries to take the sugar bowl away.

She is wearing a nautical striped T-shirt (her mom's, from the '70s), tucked into two black Wolford slips rolled down and turned into a tight, Robert-Palmer-video-style mini, and multicolored sparkly Christian Louboutin stilettos. She's just had her hair colored, returning to a sunnier shade after some experiments with both peroxide ("I woke up one morning and was like, I want white-trash hair today") and the dark side (an auburn-haired near-Goth moment last year). She's carrying a large black fringed leather Prada tote — she doesn't do small bags — and her fingers are covered with rings, most notably two vintage coiled gold snakes stacked on top of each other. ("They remind me of twins, sort of double headed.") Altogether, the effect is less her famed "bag-lady chic" than an edgy, body-conscious, and, yes, sexy silhouette. If she weren't 21, she could be 40. And French.

Few people need reminding that Mary-Kate — with her twin sister, Ashley — literally crawled into celebrity aged nine months (courtesy of Full House) and has not been out of the spotlight ever since. She has been a celebrity for more than two decades. Perhaps that's one reason she seems as if she came out of the womb worldly, the textbook old soul. "Yeah," she says with a small shrug. "I get that a lot."

With all of that attention and all of the money (her and Ashley's company, Dualstar, has famously become a "billion-dollar business"), Mary-Kate could easily have ended up the type who wears pink terry cloth and carries a variety of small dogs. "Could you imagine?" she says with the politest version of a snort. "No way." She credits her exceptionally close-knit family (she has five siblings) and, interestingly, early stardom with helping her keep her perspective. "I think it helped that I started in front of the camera, so it didn't come as a shock. If I was a teenager and was thrown into the spotlight, I don't know how I would react, to be honest."

Though the tabloids are all too keen to brand her a skinny, nervous deer in the headlights, in person Mary-Kate is easy in her skin, confident and surprisingly tactile, curling up in her seat and touching you on the arm to make a point. She laments the generic style of most actresses and cites only men as style inspirations: "Heath Ledger, Johnny Depp. Men, they just dress the way they want, and they don't think about Who Wore It Best." She doesn't much care for Who Wore It Best, noting she avoids those pages by "wearing vintage so often. I just dress the way I feel instead of looking for what's the new handbag."

If Mary-Kate and Ashley have their way, more people will be wearing clothes and carrying bags the way they do. They have just shown the fifth collection of their ready-to-wear line, the Row, and recently launched a contemporary label, Elizabeth and James, named after a sister and a brother. The Row's holiday collection (in stores next month) is a slick mix of skinny leather pants, razor-cut blazers, butter-soft, slouchy tees, and a destined-to-be-cultish pullover fur. Lauren Hutton, who stars in the Row's Spring '08 look book, says, "The clothes are extraordinary. A man I was with just loved them. The pieces are just so genius, soft like a baby's skin. Simple minimalist stuff, but really spectacular."

Mary-Kate, designer, faces an interesting challenge. She has to marry Dualstar — which has made its fortune selling tween-tastic DVDs and pastel Mary-Kate and Ashley T-shirts at Wal-Mart — with her increasingly edgy and subversive taste. Dualstar executives, some of whom have worked with her since she was a child, often nag her, mom-style, about pulling her hair back "or wearing a color," she says with a laugh. "I had this event recently, and I was like, They're going to be so happy that I'm wearing ... purple. I actually have to think about those things, though, you know, so I don't get trashed."

Get trashed sometimes she does. Hutton says, "Once in a while, she'll wear something and I'll think, Oh, baby doll, take another look. But to have the bravery, to take the chance to do that, is pretty wonderful. She is making her own way, which is hardly ever done in Hollywood." Of Mary-Kate's penchant for gigantic Balenciaga heels, Jenji Kohan, the creator of Weeds, says, laughing, "I'd be like, 'It's Tuesday. Do you really want to be wearing those shoes?' But she pulls it off." Designer Giambattista Valli, a friend, says, "She likes to take risks, but because she has such strong personal style, she always manages to make it work. Even if she had nothing on, she'd have style."

And MK chic is spreading. "Sometimes I'll look at people or at a magazine and I'll do a double take because I'm like, Oh, my God, that's my outfit, but that's not me," Mary-Kate says. Playing with her wire-rimmed aviators, she jokes wryly that she should have bought shares in Ray-Ban. (She and Chloë Sevigny pretty much brought back white '80s Wayfarers.) She tends to fall in love with a look, then wear it until she's done. "If I put together a good outfit, I'll wear it for three days and then switch it up with a blazer," she says. "I still love my vintage jeans, my tights, and my pants, though." She didn't start wearing heels, in fact, until a couple of years ago: "I kept watching Ashley walk around in them so gracefully, and I'm such a klutz. But I ended up loving heels, and I don't usually take them off." She wears precisely one pair of flat shoes: Chanel's knee-high patent-leather gladiator sandals.

This season, it's Balenciaga's fall collection — all of it — that has Mary-Kate obsessed. She is close to designer Nicolas Ghesquière and says, "He is so talented, but he's the nicest, most down-to-earth guy, and that makes everything he does more brilliant. I bought everything, but I haven't got anything yet," she says like a girl impatiently waiting for Christmas. Will she wear the new pieces with her infamous clodhopper boots? "Uh-huh. Wore them the other day, actually."

Mary-Kate always goes with her gut, even if some people (back to those tabloids) don't quite get it. "The tabloids say things about me? What do they say?" she asks archly. "People are going to write what they want, and everyone's going to have their own idea of who I am. But I'm not trying to be friends with the people who are reading them, really." After a rough couple of years filled with near-forensic scrutiny of her weight, she'll have you know that she does eat. "This is not going to sound good," she laughs, "but I like making crispy tofu sticks with peanut sauce. I love my sashimi and my salmon and my vegetables." She observes, "Stress plays a big role in how I look day-to-day. I've always been very active — Pilates, yoga. I grew up horseback riding every day for hours. I love dancing. I usually last longer than anyone on the dance floor."

A common image of Mary-Kate has her emerging from a coffee joint with an oversize cup. "I always get creamed for having my Starbucks cup," she says, sighing. "But the only time people get photos of me is when I'm getting coffee, when I can't sneak away from the camera." She also resents the pictorial implication that she and Ashley are dilettantes. "They take photos of us going into our offices, and it's 'Mary-Kate and Ashley shopping again.' But I'm going to work for eight hours, and we're working so hard. ..." She trails off. "It just shows how people want to think of you."

Mary-Kate is not above celeb watching herself, however. Newly obsessed with Victoria Beckham, she notes she avidly watched Beckham's Coming to America documentary: "She's running around in a bikini and heels, and I'm like, Oh, my God! I do that, too!" How positively Grey Gardens. "I run around my house naked with heels all the time. It's so funny. All my friends will tell you I love running around in kimonos and jewelry or naked with jewelry."

More people will be watching Mary-Kate soon, thanks to her role in the Emmy-nominated Weeds. "I am a very good Christian girl," she says with a wink. "She has her moral beliefs — and she happens to smoke pot." Of her newest cast member, Kohan adds, "Mary-Kate is complicated. She's a big celebrity, a huge media icon, but you have to separate the media images from someone who has the same issues, the same desires, as anyone else."
Of course, Mary-Kate's image, in all its incarnations — from high fashion to small screen — is her strongest asset. And she has yet to settle on one. "I feel like I've lived 10 different lives already and I'm only 21," she says, almost as a reminder to herself. "But I also feel like I'm entering a new chapter."

One thing on which she is clear, though: She doesn't need to be looked at all the time. What would she do for a day if she were invisible? "I would probably go to a restaurant with my friends, who would be able to see me, of course," she adds pragmatically, "and I would sit outside and enjoy a nice lunch with them. Then I would walk down the street."
The old soul takes a sip of her little-girl-sweet hot chocolate. "That's what I would do."