Cartoon friends strike a blow for Japanese women

On the way, she meets her soulmate: a tattooed, chain-smoking punk-chick in ripped jeans and studded leather, also called Nana (Osaki), who won't stop until she is the country's biggest rock star.

Despite sharing little in common except a name, they bond, share a flat and fight for love and respect in the world's largest metropolis, their friendship often stumbling but ultimately enduring conflicts over career, family and boyfriends.

Such is the bare plot of one of Japan's biggest pop-culture sensations - a "manga" (cartoon) story with 27 million copies in print over 13 volumes.

The Nana series has spawned novelettes, foreign translations, CDs, a looming US release and a movie, which is currently in post-production. Shibuya, the trendy Tokyo shopping district, regularly hosts youngsters sporting the Nana look - punky or demure - depending on which character they identify with.

Even in a country boasting millions of adult comic-readers and a manga market worth over 500 billion yen (£2.5bn), with plots dealing with everything from corporate battles to suicide, the success of Nana has stunned the publishing world. "We've never seen anything like it," said a spokeswoman for the publisher Shueisha, which has just released the much-awaited Volume 13.

The spokeswoman said she had "no idea" why the comic had struck such a chord, but Nana's author, Ai Yazawa, gave a clue in an interview with a weekly magazine. Explaining that she wrote Nana because she wanted to try to help women make it through their "difficult" twenties, Yazawa said: "Realising that you are not alone with your pain and self-doubt can be a source of comfort."

She added that she saw her two characters - the dreamy romantic Komatsu and the tough but insecure Osaki - as extreme examples of the same modern Japanese woman.

Not that Yazawa is touting social realism, or even feminism. Like most characters in the "shojo" (girl's comics) genre, the Nanas are cloying, baby-eyed caricatures more concerned with clothes, make-up and the mysteries of the XY Chromosome than attacking the citadels of male power; the magazine that serialises Nana, Cookie, is a shop window for beauty products.

Still, the two Nanas endure traumas well outside the limits of typical shojo themes: for much of the early series, Komatsu is stranded between the affections of two men before becoming pregnant and deciding to get married. Her friend must resist pressure from her lover to have a child so she can pursue her career. Sex, contraception and deadbeat boyfriends provide the bitter grit for what might otherwise be a very sweet confection.

Yazawa has tapped into women's concerns about their role in a changing Japan. Japanese women in their twenties are rebelling in unprecedented numbers against the traditional narrow roles assigned to them, working longer and putting off marriage, often until it is too late to have children.

Like the drama played out in Nana, many women are torn between a childless career and life at home with an overworked salaryman. In their twenties there is still space to fantasise about putting this choice off forever, finding the sort of companionship they crave in a female friend instead.