Best man Ryuichi Ichinokawa took his place before the assembled wedding guests, cleared his throat and for the next few minutes spoke movingly about the bride and groom. But his speech omitted one crucial fact: that he knew the beaming couple only marginally better than the waiters and waitresses serving their wedding breakfast.

From the moment the guests sat down until they belted out the final karaoke song of the evening, Ichinokawa was part of a grand, though well-intentioned, deception.

He is a professional stand-in, part of a growing service sector that rents out fake spouses, best men, relatives, friends, colleagues, boyfriends and girlfriends to spare their clients' blushes at social functions such as weddings and funerals.

This weekend he adopted yet another guise, as the uncle of a 12-year-old boy and his younger sister at a school sports day. He dutifully cheered them on, recorded their efforts on his handheld video camera and joined in the adult-and-child races.

If anyone asked, he would introduce himself as the children's uncle, perhaps engage in small talk, then discreetly slip away. He is unlikely to ever again set eyes on his "nephew" and "niece", or his "sister" – a divorcee whose children were being bullied at school about their absent father.

Ichinokawa launched his Hagemashi Tai [I Want to Cheer You Up] agency three-and-a-half years ago, after abandoning plans to become a qualified counsellor.

After a successful debut making the wedding speech, the requests came flooding in, says Ichinokawa, who takes days off from his job at a toy manufacturer to go on assignment. "People wanted women, old and young people, all sorts, but of course I couldn't play all those roles myself."

The affable, bespectacled 44-year-old now employs 30 agents of various ages and both sexes, across Japan with the skills and personality to temporarily adopt a new identity: as the father of a boy who is in trouble at school, for instance, or the parents of a woman attending a formal match-making party.

The number of rent-a-friend agencies in Japan has doubled to about 10 in the past eight years. The best known, Office Agent, has 1,000 people on its books.

The rise of the phony friend is a symptom of social and economic changes, combined with a deep-seated cultural aversion to giving personal and professional problems a public airing.

In recent months demand has surged for bogus bosses among men who have lost their jobs; for colleagues among contract employees who never stay in the same job long enough to make friends, and from divorcees and lovelorn singletons.

Ichinokawa's agents charge a modest 15,000 yen (£100) to turn up at a wedding party, but extra if they are asked to make a speech or to sing karaoke.

His preparation is exhaustive, examining every possible question that, if answered incorrectly or not at all, will embarrass his client and ruin his reputation. "In three and a half years I've never once been caught out," Ichinokawa says.

He even managed to keep his wife in the dark about his extra-curricular activities until two months ago, when she spotted him in a cafe being interviewed by a Japanese reporter.

"If I'm pretending to be someone's husband, I make sure I know everything about my 'wife', from her mobile phone number to what 'our' kids have been getting up to lately," he adds.

His next big assignment is to rescue a faltering love affair. The client, a woman in her 20s who is in a long-distance relationship, fears that her "good-looking, popular" boyfriend's interest is waning.

When they meet for a date next month, an equally handsome male fake "friend" will bump into her, tell her how happy he is to see her again and, if all goes to plan, prompt the boyfriend into a jealousy-driven show of affection.

It sounds improbable, but Ichinokawa is certain he will save his client's relationship. "I don't make any money out of this," he says. "But I love helping people with their problems and making them happy. When they email me afterwards to say thank you, I feel fulfilled."