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This answer contains spoilers… (view spoiler)[I'm assuming you mean Claudia. The book said she was running away because she was the oldest child and the only girl in the family which meant she had more responsibilities and chores than her younger brothers. However, the author pointed out a deeper reason for Claudia to run away. She was tired of being the perfect daughter and student and wanted an exciting challenge (e.g. running away and staying at a famous museum). (hide spoiler)]