Fortunately,(or not, depending on who you talk to) Yuffie is a secret character. She isn't needed to finish the game, but she is needed to get certain materia and items. You can find her in the forests of the first continent, on the way to Junon i think. You have to beat her to get her to join you, then answer a bunch of questions correctly and always keep your eyes on her. If you don't, she steals your money and runs like hell.
[Personality]
Yuffie really can't be trusted all that much, since she is a thief. She has a personality that makes you want to strangle her sometimes. She is very annoying, especially at one point in the game where she steals your materia. You have to chase her all around Wutai. Then, you can't find her anymore, and instead, you find the Turks. They can't find Elena, so you all join forces until you find them both. Because of Yuffie, you have to fight a boss without any magic ><.
Her attacks are good, so is her magic.
[The Truth]
Warning! This section explains nearly everything about Yuffie. If you don't want to know, then don't read!
