Critical: Shell

Critical: Shell, also known as Magic Shell, Barrier, MagiWall, and SOS Shell, is a recurring support ability in the series. It grants the Shell status as long as the user is in Critical HP or below a HP threshold.

Final Fantasy V
Magic Shell, originally known as Barrier in the RPGe translation and MagiWall in the PSX version, is learned for 10 AP from the Mystic Knight class, and can be used by the Freelancer class without having to equip it.

Final Fantasy VI
Critical: Shell is the innate ability of the Barrier Ring. The status will remain even after the user has left critical HP and will act as normal.

Final Fantasy X
SOS Shell is an ability that can either be obtained on dropped armor or customized onto armor by using 8x Lunar Curtain, which adds 4,000 gil to the armor's sell value.

Final Fantasy X-2
SOS Shell is an ability only usable through the use of the Selene Guard Garment Grid, and will grant the Shell status as long as the user is below 33% of their Max HP.