personality
Three years ago, Yuffie was Queen of what would be the land of obnoxious brats. She was selfish, and still learning about the world and about what type of person she would become. She was so absolutely sure of herself that she would boast about the impossible, creating an image of a girl that she couldn't have possibly been at only sixteen. Joining AVALANCHE was the biggest turn in her life, and has helped to make her who she's become today.
These days, Yuffie has devoted herself to helping Reeve and the World Regenisis Organization. She's grown from being selfish to caring about the people around her (and not just Wutai), even if they don't give a proper thanks in return. She'll joke about the lack of gratitude and complain halfheartedly, however she becomes embarrassed if somebody does in fact thank her for a job well done. It's a far mile from the bratty teenager that cared only about herself and what she could gain out of doing something. In fact, she prides herself in becoming a hero, and openly calls herself a champion ("of the earth and the sky"), the single white rose of Wutai, the conqueror of evil, and whatever other silly titles that could possibly come to her. With this is mind, Yuffie is clearly confident in herself and her skills as a ninja, though sometimes her grace falls short - luckily at only the less crucial times.
As certain of herself as she is, a glaring trait and weakness of hers is her lust for Materia. She claimed to want it to help restore Wutai, but even before this she has always longed for them. As she's grown she's become less likely to steal it from innocent people, but her love for Materia has never died, and most likely never will. The second and most notable weakness - and a long running joke - is her motion sickness. While in her earlier years she had to depend on a Tranquilizer to calm her turning stomach, Yuffie has gotten a somewhat better hold of this. At least now she can join the rest of the party as opposed to curling up in some corner by herself.
Fortunately, Yuffie bounces back from this among other things rather quickly. It's not absolute whether she simply holds things in or gets over them completely, but Yuffie has a tendency of shifting from anger or unhappiness to excitement and laughter in a matter of seconds should she be given the time to do so. She treats life, much like everything else, rather lightly and as a joke if she can, however the death or suffering of another is taken more seriously possibly because of her view on wars and fighting. She dislikes anything that tries to ruin the happiness of others, and is vehement about keeping the peace alive especially because of the people (Aerith) that died for it.