Sub characters join your party throughout the game and can aid you party a great deal. However, all of these Sub Characters only are in your party for a rather short time being; so even if they do help occasionally - you shouldn't rely on them. All Sub characters randomly help in battle using a spell or attack. Each Sub Character has two moves they can preform in battle.

Aero : Deals wind damage.
Cure : Restores a small amount of HP.

Attack : Inflicts damage.
Fire : Deals fire damage.

Attack : Inflicts damage.
Thundara : Deals a great amount of lightning damage.

Protect : Raises party's defense.
Cura : Restores a large amount of HP to party.

Aero : Deals wind damage.
Confuse : Inflicts confusion status ailment.

Flare : Deals damage by deteonating a tremendous amount of fire and energy.
Confuse : Inflicts Petrifies target.

Haste : Increases target's agility.
Holy : Smites an enemy with divine light.