Meet Princess Aiko, the only child of Japan's emperor who can't inherit his title because of her gender

• Princess Aiko is the only child of Emperor Naruhito and Empress Masako of Japan.
• Following her 20th birthday on Wednesday, Aiko will celebrate a series of coming of age ceremonies.

Princess Aiko, daughter of Japan's emperor, will officially come of age days after she turns 20 on Wednesday, the Japan Times reported.

The only child of Emperor Naruhito and his wife Empress Masako, Aiko currently studies Japanese literature at Gakushuin University in Tokyo, according to Royal Central.

With her birthday falling in the middle of the school week, traditional coming of age ceremonies are set to take place on Sunday, the publication added.

According to Japanese online magazine Nippon.com, Aiko will hold a press conference in March 2022 to speak about reaching adulthood, which under the current civil code in Japan takes place at the age of 20.

While the family prepares to celebrate, Aiko's transition into adulthood could serve as a stark reminder of a key issue the Imperial Household of Japan is grappling with: it's shrinking.

Princess Aiko's gender means she will never be Empress of Japan

Japan's royal family faced perhaps the most scrutiny in its modern history in the leadup to Aiko's cousin Princess Mako's wedding on October 26. Mako's relationship with her husband, Kei Komuro, was seen as controversial because of reports his mother was involved in a financial dispute with an ex-partner.

The case led to insinuations among the public that he was a "gold digger," according to The New York Times.

The Imperial Royal Household still operates under a male-preference primogeniture system, meaning only male heirs can ascend to the Chrysanthemum throne. By contrast, female royals in Japan are required under the country's 1947 Imperial House Law to renounce their title and position in the family completely when they marry "commoners," The Japan Times previously reported.

Since World War II, the number of royal family members plummeted from 67 to a mere 17, and only three heirs remain in the line of succession: Mako's father Prince Fumihito, 55, his son Prince Hisahito, 15, and Prince Hitachi, the 85-year-old uncle of the current emperor, Bloomberg reported.

The system is endangering the future of the Imperial Household, a crisis felt most by conservatives and those who see the imperial system as "a critical part of national identity," Kumiko Nemoto, professor of management at the School of Business Administration at Senshu University in Tokyo, told Insider.

Nemoto said this unbroken male bloodline has been central to Japan's imperial system, which has also symbolized the "unquestionable importance of the norm of patriarchy in Japanese family and society."

Like her cousin Mako, Aiko's likely fate is to leave the royal family

Nemoto said female members of the Japanese royal family are expected to act feminine, subservient, and demure, while "serving to the male authority including the husband, father, and the nation."

"They are still expected to take the traditional path of wife and mother," she added.

But if Aiko follows the path, she will be forced to do so outside of the royal family. "Under the current rules, she will lose her title and place," Nemoto said.

According to The New York Times, her cousin Mako is among eight other Japanese princesses stripped of a place in the family after marriage.

Unless Japanese imperial law undergoes dramatic change, spurred on from either pressure from the Japanese public or from government-led initiatives, in which advisory committees previously discussed including women in the line of succession or allowing them to maintain titles upon marriage, according to Kyodo News, Aiko's fate is sealed.

"If Japan becomes more gender-equal, the public view of and expectation to the Japanese imperial family will change perhaps to be more democratic," Nemoto said.