Who is Crown Prince Frederik, Denmark's soon-to-be king?

As a teenager, Denmark's Crown Prince Frederik felt uncomfortable being in the spotlight and wondered whether there was any way he could avoid becoming king.

Such doubts have been swept aside as the 55-year-old takes over the crown on Sunday from his mother, Queen Margrethe II, who is breaking with centuries of Danish royal tradition and retiring after a 52-year reign. The crown prince will become King Frederik X.

An athlete, decorated military officer and rock music fan, Frederik has a more informal style than his mother but is equally popular in the Scandinavian country.

Margrethe stunned the nation when she announced during her traditional New Year's Eve speech that she would step down, citing health reasons. A royal palace spokeswoman told the Berlingske newspaper that the queen informed her sons about her decision only three days before the announcement.

The crown prince, whose full name is Frederik André Henrik Christian, was born on May 26, 1968, as the first child of Margrethe and her husband, Prince Henrik, who died in 2018. His younger brother, Prince Joachim, was born in 1969.

Shy and reserved as a child, Frederik felt uneasy about the idea of ascending the throne, and the intense attention and scrutiny that come with it. In a 1996 interview with Berlingske Tidende, the crown prince said that in his teens he sometimes wondered whether he could escape his fate.

"I thought it was too uncomfortable," Frederik said. "You knew you were going to be so public, so known, so accessible to everyone and so depicted. I didn't like that."

As an adult he grew more comfortable with his position as heir to the throne and prepared himself for it. Frederik, who in addition to Danish speaks English, French and German, graduated from Aarhus University with a master's degree in political science. His studies included two semesters at Harvard University.

He received military training in several branches of Denmark's armed forces including as a frogman in an elite naval unit. Frederik got the nickname "Pingo" because of an episode when his wetsuit was filled with water and he waddled like a penguin, he said in a 2010 interview with Danish public broadcaster DR.

He learned about diplomacy during postings at the Danish Embassy in Paris and at Denmark's United Nations mission in New York. Like Britain's King Charles III, he has shown a special interest in climate change and other environmental issues.

At home in Denmark, Frederik is known for being informal and down-to-earth. While he attends official functions in medal-studded uniforms, he can occasionally be spotted blending in with the crowds riding a bicycle with his bodyguards in tow or high-fiving teenagers.

A keen sportsman, Frederik has participated in six marathons, one Ironman triathlon and a dog-sled expedition in northern Greenland. For several years he was Denmark's member of the International Olympic Committee.

Some Danes feel so comfortable around the crown prince they address him without his royal title.

Lars Hovbakke Sørensen, a historian and expert on the Danish royal house, said Frederik has a more contemporary style than his mother, effortlessly engaging in casual conversation with regular people, though he is less astute than the queen in delivering formal speeches.

While Margrethe is known for her appreciation of classical music, ballet and art, Frederik is more into rock music and sports, and younger generations find him relatable, Hovbakke Sørensen said.

"People expect the monarchs to be more informal, to be more like themselves. They don't want this very large distance to the royal persons as we have seen before," he said.

Laura Larsen, a 27-year-old social housing employee in Copenhagen, said she believes Danes will like the new king.

"He is going to be quite a relaxed king compared to our current monarch, compared to what we're seeing," she said.

Like many of his contemporaries in Europe's royal houses, Frederik found his spouse outside the aristocracy. He met Australian-born Mary Donaldson, the daughter of Scottish immigrants, in a bar during the Sydney Olympics in 2000. They married four years later.

The couple have four children: Prince Christian, 18; Princess Isabella, 16; and 13-year-old twins Prince Vincent and Princess Josephine.

Once Margrethe has abdicated, Christian will take over the title of crown prince and become first in line to succeed his father.

Mary will become queen of Denmark. Her unlikely journey from the island of Tasmania to become the world's first Australian-born queen on the other side of the world has captivated Danes and Australians alike.

At a Jan. 3 reception for foreign diplomats in Copenhagen, Australia's ambassador to Denmark, Kerin Ann Burns Ayyalaraju, said Australians are "incredibly proud" of Mary, who often has been described in magazines as a fashion icon.

"It is incredible to see how she has become such a significant part of Danish society," the ambassador said.

When Frederik turned 50, Mary praised her husband in a witty and romantic speech in fluent Danish.

"You have always pushed the boundaries, and you have insisted on shaping the world around you to fit the person and have not allowed the structures in that world to define you," Mary said.