The Princess of Wales is typically poised and controlled when she steps out in public.
Before she married Prince William, however, Kate was known to be spontaneous and outgoing - and this was certainly the case at her first ever appearance with the Royal Family in 2006.
Kate, who was dating William after the pair met at St Andrew's University in 2001, joined the late Queen Elizabeth for the prince's passing-out parade at the Royal Military Academy Sandhurst on December 15 that year.
Having officially graduated (or 'passed out') from the Commissioning Course at Sandhurst, William was now set to join the Household Cavalry's Blues and Royals as a second lieutenant.
Wearing a scarlet jacket and black Philip Treacy hat, Kate, then 24, took her front row seat and watched with pride as William marched alongside 223 newly commissioned military officers before the monarch inspected them.
Kate, who was not used to the spotlight, seemingly forgot there were cameras trained on the woman who would become Britain's future Queen after she was captured making a rather cheeky comment about her beau.
According to ITV's lip reader, Kate candidly quipped, 'I love the uniform - it's so, so sexy' before breaking into fits of giggles.
It came shortly after Kate allegedly declined an unprecedented invitation to spend Christmas in Sandringham with the Royal Family.
It was unusual that Kate was asked to join the royals in Norfolk because she was not married to Prince William at the time.
Despite the offer, Kate - who was working as an accessory buyer for Jigsaw at the time - refused Prince William's request and said she'd prefer to wait until 'she had a ring on her finger'.
Writing in her book The Palace Papers, royal author Tina Brown revealed: 'Kate, reflecting the confident mood of her Philip Treacy hat, maintained her resolve that she would not go to such a significant Royal Family gathering unless she had a ring on her finger.
'She declined. William met her decision with silence.
Instead, Kate spent Christmas with her parents, Carole and Michael Middleton, at a holiday rental in Perthshire that year.
Prince William was supposed to join them on New Year's Eve - but failed to show up at the last moment.
According to Ms Brown, William decided against driving 500 miles to see the Middleton's after a 'heart-to-heart' conversation with his father, then-Prince Charles, and the late Queen when they were at Sandringham.
Charles allegedly advised his eldest son that it was 'unfair to expose Kate to so much press harassment unless an engagement was imminent'.
It came shortly after Kate allegedly declined an unprecedented invitation to spend Christmas in Sandringham with the Royal Family and said she'd prefer to wait until 'she had a ring on her finger,' according to royal author Tina Brown
The Queen agreed, and was said to have 'favoured waiting rather than jumping'.
'It is an understatement to say that the Queen was skittish about any marriage that might fail,' wrote Ms Brown.
Yet for Kate, William's no-show was a 'humiliating' blow before they briefly parted ways in April 2007.
They reconciled less than three months later, when Kate issued an ultimatum: if they were to stay together, she wanted his full commitment. Otherwise, the relationship was over.
For the next three years, Kate spent Christmas with her family at their home in Bucklebury, Berkshire, while William would make the annual trip to Norfolk with the royals.
Kate's absence from Sandringham was not unusual - but the rule that unmarried partners are not invited has been bent before.
Meghan Markle, then newly engaged to Prince Harry, made the trip to Norfolk in December 2017.
Meanwhile, the first time Kate attended the Firm's Christmas celebrations was in December 2011 - eight months after her fairytale wedding to William at London's Westminster Abbey.
Prince William was supposed to join the Middleton family on New Year's Eve - but failed to show up at the last moment
Ms Brown said that during his time at Sandringham in 2006 (pictured), now-King Charles advised William that it was 'unfair to expose Kate to so much press harassment unless an engagement was imminent'
All eyes were on the new princess as she stepped out with the Royal Family for the Christmas morning service at St Mary Magdalene Church.
Kate also faced a particularly challenging task: what to buy her new grandmother-in-law, the late Queen, for Christmas.
Speaking on an ITV documentary to mark Her Majesty's 90th birthday in 2016, Kate revealed that her thoughtful gift had been lovingly inspired by her own grandparents.
She recalled: ‘I can remember being at Sandringham, for the first time, at Christmas. And I was worried [about] what to give the Queen as her Christmas present. I was thinking, “Gosh, what should I give her?”’
She continued: ‘I thought, “I’ll make her something”, which could have gone horribly wrong. But I decided to make my granny’s recipe of chutney.’
The late Queen relished Kate’s gift as the princess admitted she was delighted it had been placed on the dining table.
Kate added: ‘I was slightly worried about it, but I noticed the next day that it was on the table.
‘I think such a simple gesture went such a long way for me and I’ve noticed since she’s done that on lots of occasions and I think it just shows her thoughtfulness, really, and her care in looking after everybody.’
The first time Kate attended the Firm's Christmas celebrations was in December 2011 (pictured) - eight months after her fairytale wedding to William at London's Westminster Abbey
Almost two decades later, Kate paid a sweet tribute to her beloved husband at a Remembrance Day service in London - wearing the same Philip Treacy hat she had debuted in 2006 (pictured)
Almost two decades later, Kate paid a sweet tribute to William by re-wearing part of her iconic outfit from the prince's passing-out parade in 2006.
At a Remembrance Day service in London in November 2023, the Princess donned the same Philip Treacy hat she had debuted at William's Sandhurst graduation, complete with a black bow.
The timelessly elegant outfit served as a key indicator of just how far the princess had come since her first public appearance with the Queen.
This time, her face gave away nothing other than pride as she watched William, wearing his military outfit, lay a wreath at the Cenotaph.