TOKYO (REUTERS) - Japan's Princess Mako will marry a commoner in a subdued ritual on Tuesday (Oct 26) after a three-year engagement plagued by scandal and media speculation, which has left the 29-year-old niece of the emperor with post-traumatic stress disorder.

She will become an ordinary citizen after marrying Mr Kei Komuro, a 30-year-old Japanese law graduate who lives in New York, in line with laws mandating female imperial family members abandon royal status.

Their Tuesday wedding will mainly consist of filing paperwork, then holding a news conference.

While marrying out of royalty is not uncommon in Japan, the lack of pomp for a royal wedding is. Princess Mako even turned down the usual US$1.3 million (S$1.75 million) payment given to women leaving the family.

An engagement first cheered by the Japanese people soon became troubled as tabloids reported a money scandal involving Mr Komuro's mother, prompting the press to turn on him.

In the absence of clear explanations by the Imperial Household Agency (IHA), which runs the family's lives, the story spread to the mainstream press, usually scrupulous in royal reporting.

"British royals have been pretty clear when they needed to explain things, but ultimately this wasn't ever clarified," said Associate Professor Hideya Kawanishi of Nagoya University.

The saga began quietly enough in 2017 when the two college sweethearts announced their engagement.

"I'll be happy if I can make a warm and comfortable family full of smiles," Princess Mako told a news conference, with the loving looks they exchanged captivating the nation.

But just months later, the tabloids reported a financial dispute between Mr Komuro's mother and her former fiance, with the man claiming mother and son had not repaid a debt of about US$35,000. Mr Komuro has said the money was provided as a gift, not a loan. In April this year, he issued a 24-page explanation and also said he would pay a settlement.

In February 2018, the marriage was postponed until last year, ostensibly for more time to "prepare". Six months later, Mr Komuro left for Fordham University's law school, to return only three years later.

"The royal family should exist without troubles connected to money, the economy, or politics," said Professor Akinori Takamori, a lecturer at Kokugakuin University in Tokyo.

"Morally, the Japanese people want them to be impeccable."

Princess Mako's father, Crown Prince Akishino, told a news conference in 2018 that without solving the financial issue, the marriage couldn't take place, adding he and his daughter "don't speak that often recently".

He grudgingly gave in after Princess Mako issued a statement saying marriage was "a necessary choice" in November last year.

'Unblessed marriage'

Mr Komuro returned to Japan in September as a Fordham graduate and an employee at a New York law firm, but his casual ponytail caused a media frenzy as it was deemed "disrespectful".

He visited Princess Mako's parents earlier this week in a dark suit and tie, ponytail shorn. Tabloids still sniped he arrived late due to traffic jams.

After their Tuesday marriage, Princess Mako - who has never had a surname or held a passport before - will prepare to move to New York.

While their story has evoked comparisons to Britain's Prince Harry and Meghan Markle, who stepped down as senior royals last year and moved to the United States, Prof Takamori cited crucial differences.

"There isn't a place for Komuro in Japan, and so Mako, despite affection for her family, can't stay. It's not that they've fallen out with her family."

Ordinary Japanese have mixed feelings, opinion polls show.

"As a father of daughters, I think it must be quite painful for her father to recognise an unblessed marriage," said dentist Yoshinori Okabe, 63.

But Ms Chiaki Kadota, 29, said it was a private matter. "I personally think it's better to leave them alone," she said.