There's a simple way to get just the one girl in a .girlsx file. Open the file you have in the editor, select Venus, clear the list, and then add Venus to the list and save. Voila, 1 girl in the file without deleting hundreds or thousands of other girls first.
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Oh, and save the file under a different name too - wouldn't want to have an oopsie and wipe out your girls collection >;o)