In this undated photo released by the Duke and Duchess of Cambridge on Sunday, July 21, 2019, Picture: AP

Prince George likes to keep his thoughts and emotions close to his chest, whereas his sister Princess Charlotte is "outgoing."

The six-year-old royal - who is third in line to thrown after his grandfather Prince Charles and father Prince William - prefers to keep his thoughts and emotions to himself whereas his sister Princess Charlotte, four, is more "outgoing."

A source told PEOPLE.com: "George is more reserved, and Charlotte is more outgoing. Maybe it's because he's the heir and one day he might be King.

"It's the same difference between William and Harry. Harry is much more laid-back and William is the more serious one because he will be King one day."

Although the youngsters have very different personalities, their parents the Duke and Duchess of Cambridge - who also have 16-month-old son Prince Louis together - like to treat all three of their children the same. 

An insider explained: "There's a responsibility that comes with that [being third in line to the throne] but they certainly don't treat him differently. They are sending them to the same school, which says everything."

Princess Charlotte is set to join her brother George at Thomas's Battersea in London on September 5 and she can't wait to get stuck into learning. 

The source said: "She can't wait to be with George at big school. 

"She is so excited about it all."

Prince George has attended the school for two years - having enrolled when he was four years old - and has cost his parents £38,575 in fees alone thus far. 

However, Charlotte will set the duke and duchess back £18,915 a year as there is a small discount applied when a family enrols a second child.

The school prides itself on providing a "busy, thriving and purposeful" environment which offers a "rich and broad curriculum" as well as plenty of after school activities, such as fencing, philosophy, gardening and pottery. 

Despite what it has to offer, George wasn't keen on his school when he first joined.