Defense System Ψ is an enemy in Final Fantasy IV: The After Years exclusive to Final Fantasy IV: The Complete Collection. It is part of the Superboss Lost Babil, and is faced in the third part of the battle alongside Counter System X and Control System Ω. The Defense Systems are the three nodes in the back.
The player will face three Defense Systems, three Counter Systems and a lone Control System. Every turn the three Defense Systems will use Heal on the Control System, each restoring approximately 4,000 damage. The player should destroy the Counter Systems first, then focus on destroy the Defense Systems.