Kate Middleton’s unique strength is not lost on King Charles.

As she helped the King and Queen Camilla host more than 500 members of the diplomatic corps at the annual Buckingham Palace reception on Dec. 5, the Princess of Wales, 41, worked the room, glittering in the historic Lover’s Knot tiara.

“She hugely enjoyed that evening and looked really at home,” a royal source tells PEOPLE in this week’s issue. “She knows the important role the family plays in building relationships globally on behalf of the U.K.” Adds the insider: “It is clear that she and the King are quite close — in some respects closer to him than William.”

Noting Prince William and Prince Harry’s “fractious relationship” with their father, the insider says, “It’s only been recently that William has been closer [to Charles] — and Kate has acted as the honest broker in that.”

Like other royal consorts in history, Kate has developed a toughness behind the scenes as she supports her husband and future king William. “She has this public image of being nice and smiley and, dare I say, innocuous,” royal author Valentine Low tells PEOPLE. “But she is actually strong-minded, strong-willed and prepared to fight for what she wants and what she thinks is right.”

Of her role alongside her husband, a former staffer says, “She provides William with “directional setting. She guides the family and guides him.” Adds royal biographer Sally Bedell Smith: “They are pursuing the initiatives that are important to them. They are not pushing to supplant the King and Camilla—they are an incredible enhancement.”

Kate’s steeliness came into play in the immediate aftermath of Harry and Meghan Markle’s bombshell interview with Oprah Winfrey in 2021. After the Duke and Duchess of Sussex alleged that there were “concerns and conversations about how dark” their son Prince Archie’s “skin might be when he’s born,” Buckingham Palace released a 61-word statement on behalf of Queen Elizabeth.

The statement pointedly noted that “some recollections may vary” regarding what was discussed in the interview — and Low writes in his book Courtiers: The Hidden Power Behind the Crown (out in paperback in September) that Kate played a key role in pushing that phrasing.

“She thought it was very important this did not go unchallenged. She showed real strength and determination to make things clear that there was perhaps a different truth,” Low says. “Kate thinks strategically,” he adds. “She is going to be queen one day and has the longterm interests of the monarchy and royal family at heart.”

“She takes things seriously — and thank goodness for that,” says a source close to the royal household.

Adds Simon Lewis, former Buckingham Palace communications chief and cohost of the BBC podcast When It Hits the Fan, “She’s very much seen as a player at the center of team Windsor.”