She has always been close to Kate - from providing moral support at the beginning of her relationship with Prince William to being her maid of honour at their wedding.
And now, Pippa Middleton is the favourite to become godmother to the couple's first child.
It's no secret that Kate and William's younger siblings are two of the most important people in their lives, and both have been tipped to be offered godparent roles.
Joe Little, managing editor of royal magazine Majesty, said: "It is almost a foregone conclusion that they will be godparents.
"Pippa provided a heck of a lot of moral support when the couple began dating and during the brief period the couple separated. And William has always been very close to Harry following the death of their mother."
There are several other options among their close friends, including Thomas van Straubenzee, one of the Prince's best friends since they met at Ludgrove Prep School.
William van Cutsem is another close family friend. His three-year-old daughter Grace, who nearly stole the show at the Royal Wedding when she was pictured covering her ears during the RAF fly-past, is William's goddaughter.
Little said one of Kate's close friends from school may also become a godparent.
"In the past it was traditional for the royal family to have many godparents, but I don't think they will want do things on such a grand scale,' he said.
"Whoever it is it would have been given careful consideration. It would be a huge honour to be the godparent of a future king or queen."
- DAILY MAIL