LONDON — Around midnight at the cheeky tiki Mahiki nightclub, where Prince Harry hangs out when he feels like hanging out, the bottle service crew all agree on the appeal of their resident royal.

“He’s much more attractive than Will,” says Phoetini Kountouris. “He’s a bad boy, but he’s a prince.”

“He’s fit,” says a girl who introduces herself as “Tabby,” but you can call her “Tibby,” or also “Tib.” “Fiiiiiiiiit.”

Harry, Harry. Hairy Harry. The third in line to the throne is getting singler and cuter, while Prince William is getting bald. And what a big month Harry has had!

First, the expedition — a North Pole charity ad­ven­ture, in which the prince trekked over ice floes like some Arctic Indiana Jones, wearing a heroically bright orange immersion suit.

Second, the promotion. After five years in the Army Air Corps, this week he attained the rank of captain and earned his solo flying wings.

On April 29, he will serve as best man in the wedding of the decade.

That’s his most accustomed role: William’s little brother. The guy next to the future king.

The spare.

Unfortunate title. And one that could ultimately be the key to his liberation.

The strange plight of the spare to the heir: The older they get, the more obsolete they become. Order of succession gives preference to the eldest male child, then his children, then his children’s children, leaving younger siblings spiraling further from the throne.

William is the one who is about to be married. Harry is the one who is about to be jostled.

In the old days, the spares were dispatched abroad for strategic marital alliances. (Poor Caroline Matilda, the younger sister of King George III. She was shuttled off to Denmark at 15 to wed her mentally ill cousin, King Christian VII.)

The modern role is more nebulous. “In Britain, the only constitutional role is that of the monarch,” says Christopher Warwick, Princess Margaret’s friend and biographer for the last 20 years of her life. “The other people, there’s no job description.”

In a country that sometimes debates the point of its sovereign, it’s even harder to grapple with the point of the sovereign’s kid brother. It’s been a question for generations.

Tim Heald was one of Princess Margaret’s biographers and watched her slide from second in line to the throne to deep in the double digits. “She was a rock star in her day, but she wasn’t in the end, not at all,” Heald says. When Prince Edward, the queen’s fourth child, turned 21, Margaret was informed that she was no longer needed as an official counsel of state. “She’d taken that job very seriously . . . and so she was obviously a bit miffed.”

Princess Anne, Charles’s younger sister — once second in line to the throne but now 10th — has taken a proactive stance, throwing herself into a usefulness-justifying schedule that borders on self-punishing. On a recent day, she had six public appearances, including one at the opening of a Speedo factory.

Meanwhile her younger brother, Prince Andrew, gained the nickname “Randy Andy” for his tawdry relationship with a soft-core porn actress, and more recently has been in the headlines for his friendship with Jeffrey Epstein, a financier convicted of hiring an underage prostitute.

“You hear ‘Jeffrey Epstein, sex offender,’ ” Heald says wearily, “and you just know that this is the type of person the Duke of York is going to be associated with.”

It’s a recipe for disaster. Take the much-hyped “younger sibling syndrome,” then exacerbate it with crown jewels. Sibling rivalry is expected in any family, but rarely is it so ritualized.

Every time a future monarch marries or has a baby, the whole family does a game of musical chairs, shifting importance. Prince Harry is the spare to the heir now, but in a few years, he will be the spare to the spare, then the spare spare’s spare, and eventually he will be the dusty tire buried under dropcloths in the garage, awaiting emergencies that will probably never happen.

William and Harry “were brought up with very different expectations,” says Penny Junor, who wrote books on both Charles and Diana and is familiar with the family. “William has been told from a very early age that one day he would shoulder this very important responsibility. He must always be good mannered, must always keep his nose clean.”

If Harry were a normal guy, the misdeeds that defined his young adult life would probably be psychoanalyzed away by birth order and as a desperate attempt to separate himself from his older brother.

(You know, of course, the misdeeds in reference: The costume party? With the swastika armband? His defenders argued that the party was supposed to be private, that he was betrayed by a friend with a camera. His detractors argued that he was wearing a swastika.)

Even his on/off gal pal of choice, Chelsy Davy, is the outward polar opposite of his brother’s choice. The public might embrace a commoner queen like Kate Middleton, but would they accept Chelsy Davy, an heiress/law student from Zimbabwe with bleached hair who, in paparazzi pics, always looks like she’s heading off to a party thrown by Delta Delta Delta?

Of course, if you’re having a dinner party, Chelsy Davy is going to be a far more entertaining choice than Kate. Harry, too.

The queen — ever scrutinized, always restrained — is widely known for her tight-lipped discretion. Nobody really knows what she thinks about anything.

Prince William seems to be following this path: He and his beautiful fiancee appear to be lovely and sweet people. The most exciting revelation from their single joint interview was that he sometimes assists with the cooking. Whoa there, guys.

Monarchs? Boring. Boring!

The spare? Fascinating! An endless source of intrigue and fodder.

Because they don’t have to be the official face of England, they are allowed to have personalities. And unique positions in the royal family allow them to subtly influence royal culture.

Before Princess Margaret divorced her photographer husband, Anthony Snowdon, divorce was unprecedented in the modern royal family. Her split set a precedent that the queen would never have been allowed to set herself. Now three of the queen’s children, including Charles, have divorced their spouses. “She made it possible,” says Warwick, “to say, ‘My marriage isn’t working, and I’m going to just admit that it’s over.’ ” 

Prince Andrew’s marriage to the coarse, crass Fergie probably would have been frowned upon if he were up for the throne, but because he was a few steps away, it was permitted. Seeing the royal family through the eyes of someone like Sarah Ferguson — someone you could picture buying pantyhose with — provided an intimacy with the royal family that a stuffier relationship wouldn’t have allowed.

More recently, Harry was the one to deploy to Afghanistan alongside other British troops in 2008. “William couldn’t go to the war,” says Katie Nicholl, who has reported on the royal brothers for several years. A future king “wouldn’t be able to fight on the front line.”

William has never come across as more likable or relatable than when he has given joint interviews with his brother, when they tease like brothers do in a way that cannot be faked, coached or prepared for.

Maybe the official role of the spare is to be the bridge to the king.

Prince Harry has never displayed the vaguest hint of jealousy toward his older brother and has talked fondly of his future sister-in-law. As the dust settles from this familial shift, Harry will begin what is either a slow decline or a grand coming into his own. One hopes for an ascent. One predicts ascent.

Meanwhile, Harry appears happy with his role and has spoken excitedly of his best-man duties.

He has, after all, been preparing for this his entire life.