The Duchess of Cambridge’s given name is Catherine Elizabeth Middleton. While she has been referred to as “Kate” in the media and by the public, it is not known for certain whether she prefers to be called Kate or Catherine in her personal life.
In official contexts, such as announcements or press releases from Kensington Palace, she is referred to as “The Duchess of Cambridge” or “Catherine, The Duchess of Cambridge”. In informal situations, people close to her might use either “Kate” or “Catherine” depending on their relationship and familiarity with her. Ultimately, it is up to her personal preference and what she chooses to be called in different situations.
Catherine became a member of the royal family upon her marriage to Prince William, Duke of Cambridge, who is second in line to the British throne. While she does not have royal blood, she has become a prominent member of the royal family and has taken on various public roles in support of charitable causes and other initiatives.