The two-year-old royal - the daughter of Prince William and Duchess Catherine - is said to be "polite, but also fun and energetic", and she loves to engage in a conversation, particularly in Spanish with her nanny Maria Turrion Borrallo.
A source said: "She’s very sweet and very confident - she’s always chatting away.
"She has beautiful manners."
Charlotte was all smiles as she attended her first day of nursery on Monday at Willcocks Nursery School in London, which her parents - who also have four-year-old son Prince George and are expecting their third child - are said to have hand-picked because it will be an "ideal first step" in her education.
The source added to PEOPLE: "They chose that nursery because they thought it would be an ideal first step for Charlotte’s education, and they were impressed by the team who work there."
Catherine took pictures of Charlotte ahead of her first day and they were posted on the Kensington Palace official Twitter account.
The images were captioned: "The Duke and Duchess of Cambridge are very pleased to share two photographs of Princess Charlotte at Kensington Palace this morning.
"The images were taken by The Duchess shortly before Princess Charlotte left for her first day of nursery at the Willcocks Nursery School."
A spokesperson for The Willcocks Nursery School previously said in a statement: "We are delighted that The Duke and Duchess of Cambridge have chosen the Willcocks Nursery School for Princess Charlotte. We look forward to welcoming Charlotte to our nursery in January."