I think this "Mary Sue" business has gotten a little out of hand. I think most writers steal or adapt personal traits or those of people they know to add verisimilitude to their fictional creations. If the character is a girl, she will have some traits in common with you, be it only basic gender issues. The question to be asking is, how does this character serve the story? What does she add? Is she essential? Are her attitudes believable? Fit with the story/theme? Basically, the same questions you would ask about any character or scene. Having her do triple duty (humor + love story + plot advancement, for example) is even better. I'm not saying she's superhuman; I'm saying she has a valid part to play in the story.

Now, once you've got her, who is she? What was her family background? What are her nueroses? Bad or good habits? Favorite color? All that stuff that makes a character feel real. Because it's fiction, the traits you choose to flesh out in her will be relevant to the story. For example, if Frodo is in prison, having her come from an oppressive background, or (alternatively) one totally incapable of identifying with his situation, will give her more of a motivation/conflict to get involved. I'm very big on motive. What goes on between the ears is what makes a character come alive. Am I just blabbing? Igore me. Write her, enjoy her, and don't worry about any rules. You can't write well when you're looking over your shoulder. Cheers.