What name does Anne hive her diary and why?

Ann names her diary "kitty" to personify the close friend she always wanted. It is not clear where she got the name from. Some believe “Kitty” refers to Anne’s prewar friend, Käthe "Kitty" Egyedi. Others believing that Anne borrowed the name from her favorite book series, Joop ter Heul, in which the title character’s best friend was named Kitty.