Yuffie and Vincent

Here I will tell you how to get secret characters Yuffie Kisaragi and Vincent Valentine.

Yuffie Kisaragi

Go to any forest in the world and find and fight Yuffie or "Mystery Ninja". After you beat her you'll find yourself in a place with a save point, don't use the save point, and talk to her, then she will ask you a series of questions. Answer the right ones and she will be under yourcommand until Watui.
Y. Hey you spikey-headed jerk! One more time, let's go one more time!
A. Not Interested.
Y. You're pretty scared of me, huh!?
A. ......petrified.
Y. I'm really gonna leave! REALLY!
A. Wait a second!
Y. You want me to go with you?
A. ......That's right.
Y. All right! I'll go with you!
A. ......Let's hurry on.

Vincent Valentine

Go to the Shinra Mansion in Nibelheim and go upstairs to fight Lost Number inside the safe. To get in the safe, turn the lock: Right 36, Left 10, Right 59, Right 97. You have less than 30 seconds to do this. Beat the boss and get the key from the safe, and go to the mansion basement. Go to the door-like entrance and open the coffin to see Vincent. Talk to him about your mission about Sephiroth and he'll return to sleep. Wake him again and you'll be able to name him. He'll go to sleep again, but when you walk out of the basement he will join the team.