kula shakerz 2003-03-14, 12:49 pm.
You can get Yuffie in your party during any disc. If you're in Disc 1 and are riding around in the Buggy, that will work too. The two places that she appears the most often are the forests beyond the Gold Saucer (in the Gongaga Area), and most forests in the Junon Area. After a few battles you'll fight Yuffie. The battle is easy so don't worry about it.
After this she's on you team and you can name her whatever you want.
The second hidden character in the game, Vincent can also join your party during any disc. You'll need the gold key from the safe in Nivelheim's SHINRA Mansion. Go down to the cavern leading to Sephiroth's library (it's in the SHINRA Mansion, the entrance is in the stone wall in the eastern wing). Remember the door to the crypt you couldn't get through? You can now enter it. Stand at the foot of the purple coffin and hit "O". The lid will fly off and Vincent will talk to you. Choose the bottom reply when he asks you a question. When the coffin closes, press "O" again. Choose the second (bottom) reply when Vincent poses another question, and you can name him. Then try to leave the cavern and he'll join you.