¤ How to catch Yuffie

In the forest near Junon/Fort Condor and the forest near Gongaga you can encounter a ninja woman. Defeat her and you will come on a little field. DON'T SAVE, CALL THE MENU OR GO AWAY, BECAUSE IF YOU DO, SHE IS GONE AND STEALS GIL FROM YOU!! The dialogue should now go as follows:

Yuffie: "You spikey headed jerk! One more time, let's go one more time!"
Answer: "Not Interested."
Talk to her again.
Yuffie: "You're pretty scared of me huh!"
Answer: "......petrified."
She starts to leave but then turn around and says:
Yuffie: "I'm really gonna leave! REALLY!"
Answer: "Wait a second."
Yuffie: "You want me to go with you?"
Answer: "........That's right."
Yuffie: "All right I'll go with you!"
Answer: "Let's hurry on."
Providing you followed the dialogue correctly. Yuffie will now join your party and you will be able to control her.

¤ Yuffie's Materia Quest

When you go ashore on the island where Wutai is (big island on the right of the world map, near Rocket Town) you will be attacked by some soldiers. When you defeated them Yuffie is gone and has stole your materia. Now go to the town of Wutai. Wutai is Yuffie's hometown and the team will have to play a little game of hide and seek. Equip Cloud with the Yoshiyuki sword that you got in Rocket Town. This will prove very useful as he has no need for materia slots at the moment. When you arrived in Wutai you see a glimpse of Yuffie. Go to the Large temple and enter the house on the right and rest for the night in the bedroom. When you awake you'll see a hidden passageway. At the end of the passage you will find a chest containing a Magic Shuriken. Exit the bedroom and go the library to the north of the house. At the right there's another secret passageway and you will find there a chest containing a Hairpin. Now exit the room and enter the room to the left. Speak to Godo the whole time and then Yuffie appears. Yuffie and Godo have a little quarrel and Yuffie runs away again. Exit the house and go enter the Turtles Paradise bar. Talk to Elena, Rude and Reno and they will tell you they having a little vacation. Exit the bar and enter the Item Shop. Open the chest contained MP Absorb Materia. Before the team can equip this Yuffie will steal it and runs away. Exit the shop and cross the 3 bridges and enter the house near the entrance. Look behind the screen and you will see Yuffie. Before you wants to grab her she runs away again. Exit the building and check the flowerpot near Turtles Paradise. The pot is moving a lot. The team will split and seal all exits. Yuffie agrees to give your party's materia back. Follow her to her hideout and enter it. Before entering the room check the wall for Turtles Paradise No. 6. When you enter the room Yuffie tells you to pull a lever to get your materia back. Pull the lever and you will activate a trap. Yuffie runs away. Exit the house and go to the large temple again. Before the temple go to the left and ring the gong. This will open a secret door. Enter it and you'll see Don Corneo is there and that he have kidnapped Yuffie along with Elena. Before chasing him open the 2 chests. in the next room you'll have to fight 3 soldiers. When you defeated them Don Corneo has fled. Exit the building a go to the mountains just next to the item shop. You'll see Reno & Rude that give up their vacation to get Elone back and kick Corneo's butt. Follow them until you reach a junction and take the path that heads north. Take north again at the next junction, then you'll see a cave. Enter it and open the chest near the exit that contains a Dragoon Lance. Exit the cave and follow the patch to the south. You'll find Don Corneo and his two captives. Then you must battle Rapps. When you defeated Rapps, Yuffie will give back your materia.