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.