Yuffie is the cool 16 year old Ninja that resides at the Japanese town of Wutai.  She forces her way into your party for only one purpose: to get as much materia as humanly possible.  For some weird reason in my game, she always does the most damage(Cloud only does 2,500 damage for each attack, Yuffie does more than 5,000).  Maybe my game is bugged or something.

Limit Breaks
Lv. 1: Greased Lightning-Sorry no John Travolta and leather jeans.  Weird name, but Yuffie does damage by hitting the opponent twice.
Clear Tranquil-Essentially a weaker version of Aeris' Healing Wind.

Lv. 2: Landscraper-Yuffie slams into the ground and does Earth damage to the enemy.
Blood Fest-Almost exactly the same as Seifer's Blood Fest attack in FFVIII, Yuffie slams into all enemies for major damage.

Lv. 3: Gauntlet-Essentially the same as Jenova-Life's Aqualung.
Doom of the Living-Yuffie slashes fifteen times against the enemy, dealing huge damage.

Lv. 4: All Creation-Yuffie summons a huge laser beam to strike all enemies.  To receive this, finish the Yuffie subquest in the town of Wutai.  Once you do this and get your materia back, go to the end of town, straight into the huge Japanese temple, and fight the five gods of Wutai to receive All Creation.  Remember to junction all your most powerful materia to Yuffie, and make sure she always has Bahamut equipped.

Victory Dance
I can't really explain this one..  Yuffie goes into a seizure and spins around in a circle.