In 2011, Prince William and Kate Middleton vowed at the altar to look after each other—for better, for worse, for richer, for poorer, in sickness and in health.

Thirteen years later, Kate has needed her husband to be there for her more than ever, the Princess of Wales having shared in March—following months of speculation about the true state of her health—that she was undergoing preventative chemotherapy after a cancer diagnosis.

And in a bizarre twist of fate for the systematically stoic royal family, King Charles III has also been battling an unspecified type of cancer, which until he returned to public-facing engagements in April left William as the highest-ranking public face of the entire operation.

"It's a tough time for him," royals correspondent Sharon Carpenter told E! News after the king disclosed his diagnosis in February, "because his wife is recovering, he's got three children, and he has his own responsibilities as well."

But, she noted, "the royals are very good at keeping calm and carrying on, no matter what's going on behind closed doors."

It's behind closed doors where William, who's turning 42 on June 21, has been stepping up without fanfare, taking periodic breaks from official duties since January to be home with Kate and their children, Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6.

After confirming her plan to attend only the day before, Kate and the kids made their first public appearance since Christmas on June 15 at Trooping the Colour, the monarch's official birthday celebration historically held this time of year (though Charles was born Nov. 14, and his late mother Queen Elizabeth II on April 21).

NBC News royal correspondent Katie Nicholl called it a "wonderful surprise," explaining, "We know that as the King and the Princess of Wales have gone through this cancer journey together, they've very much been supporting each other behind the scenes. Well this is the ultimate show of support."

William's young family also joined Charles and Queen Camilla on the Buckingham Palace balcony after the ceremony for the first time since last year's Trooping the Colour, which proved to be a prime opportunity to analyze the state of the royal vibes.

"You can see the difference in Kate's smile, which becomes evidently wide, as William looks straight in her eyes whilst they exchange communication on the balcony," mindset coach and body language expert Noor Hibbert told HELLO!. "His eyes show care for his wife and this affectionate exchange demonstrates the deep and romantic bond between the two."

Royal photograph James Whatling, who snapped the moment, concurred. "If you look, you can look at the minutiae of it, because you're taking 20 frames a second and so it goes from a little look," he told the publication, "and then Kate's smile broadens, and it's lovely, and it's very much a welcome back."

Whatling added, "They're obviously a very strong unit."

Father's Day also passed without controversy, Kate sharing a photo she took of William and the kids with the caption, "We love you, Papa. Happy Father's Day...G, C & L."

When she first shared her diagnosis following "an incredibly tough couple of months" for their family, Kate paid special tribute to her husband. "Having William by my side is a great source of comfort and reassurance," the 42-year-old said in her unprecedentedly candid video message, recorded on the grounds of Windsor Castle. 

In a detailed June 14 update, Kate shared that she was "making good progress" but still had good days and bad.

"My treatment is ongoing and will be for a few more months," she explained. "On the days I feel well enough, it is a joy to engage with school life, spend personal time on the things that give me energy and positivity, as well as starting to do a little work from home."

In the meantime, William's been fielding well-wishes and accepting cards and bouquets on Kate's behalf while out and about. While he's never been one to go into detail about his family's personal business (in sickness or in health), he's been ever ready with a reassuring, "We're all doing well." 

Kate shared in her latest missive that she's been "blown away by all the kind messages of support and encouragement," adding, "It really has made the world of difference to William and me and has helped us both through some of the harder times."

The Prince of Wales has also appreciated these public displays of empathy, the future king not minding a pick-me-up, either, amid such trying times.

"The double illness has placed enormous pressure on William, both as a parent and inheritor of the family business," royal biographer and The Crown historian Robert Lacey told the magazine. "It has brought all kinds of challenges. He [has handled] it with calmness and lack of drama."

And, Lacey added, "From what we know of their personal lives, for William and Kate, spending time with their children is the most precious and enjoyable thing they can do."

Kate's parents are also among the couple's support system, William joining Carole and Michael Middleton at Royal Ascot on June 19. Close to his in-laws since his days courting their daughter while they were students at University of St. Andrews, at one point William took Carole's hand as she freed the heel of her shoe from the grass at Ascot Racecourse in Berkshire.

Ahead of William and Kate's 13th wedding anniversary April 29, a lifetime since they were co-eds in Scotland, royal biographer Robert Jobson called the milestone "a testament to not only their love, but also their unity."

"I'm sure they'll acknowledge their journey, knowing that their strength lies in being together," the William at 40: The Making of a Modern Monarch author told HELLO! The ups and downs they have weathered together have cemented their bond, which is now unshakeable."

Keep reading to celebrate more of William and Kate's sweetest moments through the years:

Sometimes the date night dress code calls for tiaras and tails, as it did at this Dec. 5, 2023, reception at Buckingham Palace for the Diplomatic Corps.

Kate and William leaned in courtside when they attended a Nov. 30, 2022, Boston Celtics game during a trip to the U.S.

Still making Kate smile after all of these years, perhaps William started singing early as they headed into the "Together at Christmas" Community Carol Service at Westminster Abbey in 2021.

Queen Elizabeth II joined her grandson and his wife of then-10 years for a tour of the Chelsea Flower Show in May 2019.

The longtime loves shared a toast and a beer after attending the St. Patrick's Day parade in Hounslow in March.

While visiting Northern Ireland in February, the duo got their hands dirty during a soccer match.

During their Northern Ireland trip, the Duchess of Cambridge poured herself a pint before her hubby enjoyed a glass.

Prince William couldn't keep his eyes off of Kate while celebrating the 100th year of the Royal Air Force in England in July 2018.

On April 23, 2018 the future King of England and his stunning wife welcomed their third child, Prince Louis, into the world and of course they looked perfect posing for the cameras.

The royal couple bundled up on their winter trip to Norway in 2018 and looked adorable side by side.

The athletic duo cheered on the tennis players at Wimbledon in 2017 and they seemed to have a blast doing so.

Prince William looked on in amazement as his wife played with a bow and arrow while on their trip to Bhutan in 2016.

Ugh, could these two be any cuter?!

The happy couple waved to fans in May 2015 after welcoming their second child, Princess Charlotte, at the famed Lido Wing of St. Mary's Hospital on May 2.

When the Cambridge crew was just a family of three they had a blast visiting Australia on their royal tour.

As you can see, the lovebirds are always up for a wild outdoor activity in between performing their royal duties.

The proud new parents posed for the cameras outside of St. Mary's Hospital after welcoming their first child, Prince George, into the world on July 22, 2013.

The couple visited the Emirates Arena, the future venue of the 2014 Glasgow Commonwealth Games, in Scotland in 2013 and they were perfectly in sync while walking around.

Princess Kate looked to her hubby for warmth as the two attend the Cheltenham Festival at Cheltenham Racecourse in March 2013.

The married couple waved to onlookers as they attended the wedding of Mark Tomlinson and Laura Bechtolsheimer in Switzerland.

Even the Duke and Duchess of Cambridge get nervous when watching the Olympics...but at least they have each other.

The royal duo shared a laugh while watching the cycling events at the Velodrome during the London 2012 Paralympic Games.

The two arrived at the Honiara International Airport in a special boat—talk about the royal treatment—before exploring Guadalcanal Island in the Solomon Islands as a part of their 2012 tour.

A couple who eats together, stays together! The Duke and Duchess of Cambridge attended an official dinner hosted by Sultan Abdul Halim Mu'adzam, Shah of Kedah while on their Diamond Jubilee Tour of South East Asia in fall 2012.

The royal couple was red carpet ready as they arrived together for the 2012 celebration of the British Olympic Team GB at Royal Albert Hall.

Kate gave her royal hubby a smooch after his team won a tournament round at an event for the American Friends of the Foundation of Prince William and Prince Harry in July 2011.

The couple shared a sweet kiss on the balcony of Buckingham Palace after their wedding on Friday, April 29, 2011 and it was magical.

The Royal Wedding was pure bliss and the newlyweds couldn't contain their happiness as they left the church as husband and wife.

Prince William looked lovingly at his wife as they attended Harry Meade and Rosie Bradford's wedding in October 2010 and, aww.

The royal couple was all smiles on their graduation from the University of St. Andrews back in June 2005.

(E! and NBC News are both members of the NBCUniversal family.)