October 17, 2005 12:00 PM

Being the sweetheart of William Arthur Philip Louis Windsor is a job with a few requirements-among them good breeding, impeccable manners and the ability to pull off a silly designer hat. But as Kate Middleton knows firsthand, no quality is more important than grace under pressure. Back in June, the 23-year-old commoner made her public debut as William’s girlfriend at the wedding of a family friend, Hugh van Cutsem-and had to stroll to a quaint English country church while the prince attended to his duties as an usher. Rushed by shouting photographers, Middleton smiled the whole way, but observers say she was so badly rattled she slid into a waiting car rather than walk to the reception. Says one observer: “It was outrageous.”

But when you date a future king, it’s all part of the drill one that Middleton has since gotten down pat. Attending a horse show with her mother at the estate of Princess Anne on Aug. 6, a cool and confident Kate employed a far different tactic on swarming paparazzi: the royal brush-off. “If I let you take photos now,” she said crisply, “you’ll want me to pose at other events.” And as she maps out a new life for herself in London-looking for art gallery jobs by day and hitting the clubs with her prince by night-she’s taking the media crush in stride. “Kate is used to the photographers now,” says a friend. “She gets snapped at every event she goes to.”

Little wonder why. In the last few weeks British tabloids have been falling over each other to rush Kate and Will-who have been close since ’02, when both were in their first year at St. Andrews University-to the altar. With graduation day behind them, the pair have both landed in Britain’s capital-and seem closer than ever. Kate, an art history major, has been commuting from her parents’ home in rural Berkshire during her job search (she recently interviewed for a job at Laurent Delaye Gallery in chic Mayfair), while William, 23, is occasionally crashing at Dad’s place-Clarence House-and soon begins a banking internship while tackling his entrance exams for Sandhurst. That left the couple plenty of free evenings at nightspots like Mamilanji in Chelsea over the summer. William and his girlfriend of three years have also gone in for more PDA than during their student years. Snuggling at Purple, a Chelsea nightclub on Aug. 25, “they seemed very much into each other,” said a guest. “She was sitting on his lap for a while.”

By all accounts, the pair are an ideal match. An outdoorsy sort, “Kate loves the countryside and sport,” says someone who has worked with her. Middleton has loyally watched William play match after rugby match and, unlike many a royal girlfriend, seems to share the Windsor passion for hunting and shooting. “They are very, very close,” says a family friend. “But it is way too premature to consider marriage.” As William himself told a reporter from Britain’s Sun newspaper last spring, “Look, I’m only 22 for God’s sake. I am too young to marry. I don’t want to get married until I’m at least 28 or maybe 30.” Or, as another friend of the pair’s puts it, they’re “young kids still trying to find their feet in life. She has got to get a job and find a career, and he’s got to slip into his role as comfortably as possible.”

If William isn’t yet shopping for rings, he has made his girlfriend one of the family. The two rented a farmhouse together in their junior and senior years at St. Andrews and hosted a lunch for Prince Charles, Camilla and Kate’s parents, Michael and Carole, after their graduation on June 23. That same month, Charles-who invited Kate to join his sons and close friends on a Swiss ski holiday before his second marriage last spring-is said to have entertained the same group at his estate, Highgrove. And in July William and Kate left for a jaunt to the sprawling African game farm run by the father of Will’s friend Jecca Craig; traveling with a large group of friends, “they all had a superb time,” says a pal.

All that pales beside the biggest seal of royal approval of them all: Middleton has met the Queen, most likely at her Scottish estate, Balmoral, where the couple spend romantic breaks in a cottage set aside for William and Prince Harry. Her Majesty is said to approve: “[Kate] is very nice and very unassuming. She’s not in it for personal gain,” says a friend of the monarch’s. “She completely understands that William’s got to have his privacy, and she puts that first.”

Middleton also knows firsthand that privacy does not come easily: During William’s four years at university, the press agreed to keep its distance and not to document William’s romantic exploits-as low-key as they were. Now, however, “the protective bubble has burst,” says Vivienne Parry, former trustee of the Diana Princess of Wales fund. “Kate is going to be under intense pressure-it’s kind of make or break.”

That pressure famously tested William’s mother, Princess Diana, who found life in the royal family claustrophobic. Concerned for his girlfriend, William has asked Palace aides to help Middleton learn to cope with the pressures and his protection officers to shadow her when necessary. “I am sure people have talked to them about it and said, ‘Smile, do your bit, and in five minutes you are back to normal life-if they are going to take a picture you may as well let them,'” says a friend of the Windsor family’s.

Unlike Diana, Middleton also has a trait deemed essential for a royal girlfriend. “She doesn’t speak to anyone about [the relationship]. It is between the two of them,” adds a friend of William’s. Says the family friend: “Thank God she is so discreet.” Or, as Gemma Williamson, a friend of Middleton’s from high school puts it, “If they broke up, she would not be the one to spill the beans or rinse him for his cash.”

Chances are, however, that William isn’t fretting that Kate will take the money and run. “What is important to him is that she gets on with all his close clique of friends his age, Guy Pelly and William van Cutsem,” says a source close to the group. In turn, William’s chums are careful to look out for her: In March, when the couple ended a night of partying at Sugar Hut bar in wealthy Fulham, Eton classmate James Meade escorted Middleton past photographers.

Sudden celebrity hasn’t gone to Middleton’s head. Though her parents are worth an estimated $5 million, they aren’t ones to put on airs: Carole, 50, was once a flight attendant, and Michael, 56, heads the family mail-order business selling children’s party goods. The oldest of three children (sister Pippa is 22 and brother James 18), Middleton was raised in the genteel village of Bucklebury and was popular with classmates at her $35,000-per-year boarding school, Marlborough College. A prefect in her senior year, Middleton-then known as Catherine-was “this smiley, happy, sporty girl,” says school friend Williamson, who was on the hockey team with her. “Good at tennis, swimming, running, maths and science. She was never bottom of the class, but she wasn’t top either.” Socially, Williamson says, Middleton was generous: “She was the most kindhearted woman. There are always cliques at school, but she would have time for absolutely everyone. That’s what made her special.”

The prince, who, like Middleton, entered St. Andrews in the fall of 2001, apparently recognized her empathetic qualities; after they met through boarding school friends, she is said to have listened as he struggled with doubts about college. The press first took notice in April of ’02, when an admiring William was photographed in a prominent seat at a charity fashion show where Middleton modeled a diaphanous dress.

Discretion, however, worked to their advantage. When the prince and Middleton moved into a Victorian building off-campus in September ’02, it was with a pair of roommates and assertions that they were just friends. Though the Palace refused to acknowledge the romance when they moved into their farmhouse in the fall of ’03, classmates saw them as a couple. “They are obviously together,” said a St. Andrews student at the time.

Now, of course, Middleton’s ability to dash into Pizza Express without being photographed-as she and William often did at St. Andrews-has evaporated. “People are eager to know what she’s like,” says Parry. “A picture of William and Kate kissing is going to be worth tens of thousands of pounds.”

That may be only the beginning. While the Queen is said to be wary of an early marriage, insiders say she’s not likely to appreciate a succession of princess-bride candidates, either. “She believes it might be a good idea for William to have a companion in his royal duties, rather than an endless stream of girls chasing after him just so that they can slobber all over a prince in the way that happened to his father,” says one of the Queen’s friends.

So far, so good: “She is princess material and 21st-century queen material, I have no doubt about that,” says veteran royals writer Robert Lacey. “So far she’s not put a foot wrong. And if she is excited by what is happening to her, she is managing to hide it very well.”

Michelle Green. Simon Perry, Ellen Tumposky, Laura Hahn. Sara Hammel and Susan Clarke in London.