Crown Princess Masako flashed a bashful smile at a surprise rendition of “Happy Birthday” during a concert as she celebrated her 55th and final birthday before she becomes empress.

Crown Prince Naruhito beamed on stage on Dec. 9 at the regular concert of the Gakushuin alumni orchestra, where he played the viola. The couple’s daughter, Princess Aiko, clapped her hands delightedly in the audience.

Masako, who will become empress on May 1, when Naruhito ascends the Chrysanthemum Throne, has been out of the public eye at times due to a stress-induced “adjustment disorder.”

However, during the past year, the crown princess appeared at many events she had skipped since she went under medical treatment in 2003.

In January 2018, Masako attended the Imperial New Year’s Lectures, a court event where Emperor Akihito and Empress Michiko hear lectures from experts in academic circles. She was also present at an annual meeting of the Japanese Red Cross Society in May.

A key factor behind her recuperation and more active public life is the growth of Aiko, who turned 17 on Dec. 1, according to a source close to the family.

Aiko, along with Naruhito, appears to have become one of the closest and most reassuring sources of emotional support for Masako.

Conversations in the family have become more like those among adults than between parents and their children, the source said.

Aiko attended summer school last year at Eton College in Britain. In autumn, she was part of a dance team during a school festival at Gakushuin Girls’ Senior High School, where she also put on an impressive performance as an emcee, the source said.

During a stay at an imperial villa last summer, Aiko appeared poised and confident as she responded to questions from the media, a sign of how she has grown into a mature adult her family could depend on.

On the morning of her birthday, Masako visited the Imperial Residence to give her greeting to the emperor and the empress.

Wearing a matching light blue high-necked dress and hat, she smiled and waved in response to calls of “happy birthday” from well-wishers as her car passed through the Hanzo-mon Gate into the Imperial Palace.

In a statement released on her birthday, Masako noted that the encouraging remarks she has received from the public during her tours to provincial areas and on other occasions have been a “major support” for her.

“More often than not, the words I hear from people feel so intimate to me,” she said in the statement. “I am also so grateful for the cordial remarks I received from many people during the garden party."

During the autumn imperial garden party in November, she walked the full route for the first time in 15 years and conversed with many of the guests.

Masako’s schedule in September was something of a surprise for reporters covering the Imperial Household Agency.

She visited Fukuoka Prefecture on an overnight trip to a community impacted by the northern Kyushu downpours in July 2017, and attended the International Youth Conference in Tokyo the following day.

Imperial Household Agency officials, out of fear of exacerbating her exhaustion, had previously never assigned Masako to go out to perform an official duty following an overnight provincial tour.

The team of doctors for the crown prince’s household, who are responsible for Masako’s treatment, said in a statement released on her birthday that her steady completion of one goal after another has allowed her to regain self-confidence and further broaden the scope of her endeavors.

“I will work hard and do my best so I can be of service to the Crown Prince and contribute to the happiness of the people,” Masako said in her statement.

While she conceded, “I do feel uneasy about to what extent I will be able to be of service,” the remarks suggest her determination to take on the heavy responsibility she will bear as empress in the new era to come.

(This article was written by Yudai Ogata and Ayako Nakada.)