Who is Kitty in The Diary of a Young Girl?

Kitty is Anne's name for her diary. Anne considered her diary to be her closest friend, as her diary is the only place she unburdened herself of her deepest thoughts. Anne was also very fond of the books of Cissy van Marxveldt about a character named "Joop ter Heul", who is constantly corresponding with her many friends. Joop's closest friend in the book is a character named Kitty Francken, and this is the reason Anne called her diary "Kitty".