Nagi is the only heir to the Sanzenin fortune because she is the only survivor of the family besides her grandfather. She isn't just your average "rich girl". Calling her "rich" would be selling her short. The "Sanzenin fortune" is so large, throwing away 20 trillion yen in a red herring investment is considered a prank. Moreover, Nagi fell for that "prank" when she was a child - she's 13 now, and by far wiser. She doesn't particularly desire the inheritance. However, other people do want that wealth, and will acquire it by any means necessary, leading to many complications for Nagi, the least of which involve kidnappings and attempted assassinations. Hayate, her butler, is indebted to her, so Hayate always save Nagi from any accident, kidnapping, or assassination. Even though Hayate himself attempted to kidnap her for ransom money, Nagi has a crush on Hayate ever since she first met him in the kidnapping situation.
an extended description!