In FFVII theres 2 secret characters, Vincent Valentine and Yuffie Kisaragi i already made a instructable on getting Vincent so heres how to get Yuffie. You have to do a task to get them, a different one for each. Getting Yuffie is pretty easy so dont worry about doing to much work.
Yuffie is only available after you go to the Mythryl Mines. Once youve been there, you can find her in any forest on the world map