Beneath each character's HP and MP is an overdrive guage. When it's fully charged, the character in question cna perform a special attack called an "Overdrive". When this becomes available, press <- (LEFT) to access your overdrive(s). Guages charge whenever a character takes damage. This setting can be changed as you travel through the game.
Overdrive
Target
Dragon Fang
All enemies
Default
Shooting Star
One enemy
1 Sphere
Banishing Blade
One Enemy
3 Spheres
Tornado
All Enemies
All 10 Spheres