What do KO, Revive, and Retire mean? What do Crystals do?

■ KO/Revive/Retire
Units whose HP has been reduced to 0 are knocked out (KO'd), and a countdown appears on the screen.
When the countdown reaches 0, the unit can no longer be revived and will withdraw from the battle.

KO'd units can be revived with the "Raise" ability or with the "Phoenix Down" item.

■ Crystals
Picking up Crystals on the map can restore HP or TP. They can also grant JP.
When the countdown for KO'd units reaches 0 and they are removed from the battle, they turn into Crystals.

These Crystals can also be taken by enemies, so be sure to acquire them as quickly as possible.