Kate Middleton and Princess Diana: The Women in Prince William's Life

April 28, 2011 -- Engagement rings symbolize the love and commitment that encircles two people who have decided to join their lives forever. The ring Prince William gave Kate Middleton last November is bigger -- both in carats and symbolism.

It also symbolizes Diana -- its original owner -- and her importance to William.

This was visible from the beginning, when the two announced their engagement. "It's beautiful. I just hope that I look after it," Kate said.

"If she loses it, she's in very, very big trouble," William added.

He went on to tell reporters that the ring was his way of keeping his mother "sort of close to it all."

In this and so many other ways, she already is.

Watch the full story on 'Nightline' tonight at 11:35 p.m. ET

At first glance, Diana's and Kate's stories seem similar: two beautiful young women marrying into the royal family while the whole world watches. We remember Diana as the People's Princess -- glamorous, outspoken and compassionate -- qualities we see reflected in Kate.

But perhaps it's in their differences that we see Diana's true legacy, and its role in her son's wedding and adult identity.

As a 19-year-old bride-to-be Diana was as much a girl as a lady. Though it may be hard to believe, she had met Prince Charles only a dozen times before they married.

The pressure of public life was almost too much to bear, according to ABC News royal correspondent Katie Nicholl.

"Diana's body language was very closed. You could see she wasn't very comfortable with the cameras," Nicholl said. "You'd see her looking down and allowing the fringe [of her hair] to cover her eyes, because she didn't want to be photographed."

Her tortured relationship with the press would last throughout her life, and she died trying to get away from the paparazzi. Few who watched her funeral will ever forget the image of her sons, Princes William and Harry, solemnly following her casket through the London streets.

In an interview with "Nightline" anchor Cynthia McFadden, India Hicks, one of Diana's bridesmaids, reinforced Nicholl's characterization of Diana as an isolated girl who was in over her head. During the weeks of fittings and rehearsals leading up to her and Charles' wedding, Hicks said she never saw Diana with her mother or sisters.

"I think it probably was the sign of a woman who was at sea already," Hicks said. "She wasn't terribly grounded, even at that stage."

Kate Middleton, 29, is more sure of herself. She has benefited from having seen what Diana went through.

She grew up in what's been called a chocolate box of a British village: Bucklebury. Kate knew the butcher, the grocer and the owner of the local pub, all of whom she's invited to the wedding.

"He really chose the Un-Diana," said Tina Brown, editor of Newsweek and The Daily Beast, and author of a book on Diana. "Everything about Kate is counter-programming to his mother. I don't mean that he didn't adore his mother -- he absolutely did -- but there was so much drama, so much craziness, so much tragedy, really, in his life as a child growing up that he really needed somebody to be the anchor, not the drama, in his life."

"Even when she's just out shopping," said Nicholl, "she's smiling. She's flicking her hair, she's very aware of her presence, she's very aware of her effect on the cameras ... and she does it incredibly well. That took Diana years to master."

Because of these differences, many believe Kate will have a smoother ride into royal life than Diana did. The key to her success, beyond her ease before the public and the press, has been her ability to show the discretion demanded by the ultimate audience: the queen.

Watch a special '20/20' Thursday at 8 p.m. ET for a behind-the-scenes look at the life that awaits Kate Middleton and join us again at 4 a.m. Friday morning for ABC News' live coverage of the royal wedding with Diane Sawyer and Barbara Walters.

Diana's Lesson and Legacy

"I think that the queen knew from the start that Diana was going to be difficult, timid, shy," said Nicholl, saying that Kate -- even through two breakups with William -- has never embarrassed the royal family.

"She was the absolute essence of decorum and royalty and dignity and discretion," Nicholl continued, "and I think she earned her brownie points, and what the family have done is they've brought her in."

Jules Knight, a member of the British boy band Blake, said he was friends with William and Kate at the University of St. Andrews in Scotland, and that he is not surprised she has stood up to the pressure so far.

"I think she's conducted herself in the most graceful way," he said. "She hasn't put a foot wrong. She's not the kind of girl you'd see falling out of a nightclub. ... She's a reasonably sensible girl. She's very engaging to talk to, very down to earth and normal."

And, he said, she's the perfect woman for Will.

"She was never seen as the girl who sort of nabbed a prince or anything like that. I think if anything he was sort of seen as the lucky one. ... She's one of those girls who's practically perfect in every way, and I think that we all thought that from the moment we met her, right until now."

William and Kate have been together on and off for eight years, and they have tried to live as normal a life as possible. It's a lesson he learned from his mother.

"Diana famously said to [her sons], 'Boys, there is life outside these palace walls,'" said Nicholl.

She added that Diana took them shopping, gave them money and said they could go buy their own comic books and schoolbooks. She even took them to the movies and McDonald's.

"I think all of these things, even if they weren't aware that they were being taught -- this was Diana transferring her legacy and her humanness," said Nicholl.

This legacy shines on, in William, in his relationship with Kate and in the ring she will wear walking down the aisle and into their future.

ABCNews' Edward Lovett contributed to this report.