She will become an ordinary citizen on Tuesday after marrying 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 marriage will mainly consist of filing paperwork, then holding a news conference.

While marrying out of royalty isn't uncommon in Japan, the lack of pomp for a royal wedding is. Mako even turned down the usual $US1.3 million ($A1.73 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 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 Hideya Kawanishi, an associate professor at 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," 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 Komuro's mother and her former fiance, with the man claiming mother and son hadn't repaid a debt of about $US35,000. Komuro has said the money was provided as a gift, not a loan. In 2021, he issued a 24-page explanation and also said he would pay a settlement.

In February 2018, the marriage was postponed until 2020, ostensibly for more time to "prepare". Six months later, 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 Akinori Takamori, a lecturer at Kokugakuin University in Tokyo.

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

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 Mako issued a statement saying marriage was "a necessary choice" in November 2020.

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

He visited 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, 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 England's Prince Harry and Meghan Markle, who stepped down as senior royals in 2020 and moved to the United States, 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 Yoshinori Okabe, 63 and a dentist.

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

Australian Associated Press