SOS Shell

SOS Shell, also known as Magic Shell, is a recurring support ability. It automatically puts the user in Shell status whenever their HP is in critical, and will remain as long as the character is in Critical status, making the ability extremely risky to keep.

Final Fantasy V
Magic Shell is the Level 1 ability of the Mystic Knight class. It requires 10 ABP to learn, and will automatically activate when in Critical if the character is a Freelancer.

Final Fantasy VI
The ability can be found in the Relic Barrier Ring.

Final Fantasy X
SOS Shell is an ability that can be found on dropped armor, or be customized onto armor by using 8 Lunar Curtain.

Final Fantasy X-2
SOS Shell is an ability of the Selene Guard Garment Grid.