Auto-Shell

Auto-Shell (オートシェル) is a recurring ability that maintains a constant Shell status on the user. It is usually non-dispellable and will not wear off even if the user is knocked out and revived in the same battle.

Final Fantasy VI
Auto-Shell is found on the accessories Princess Ring and Guard Bracelet. Princess Ring is only equippable by females and Guard Bracelet adds the Auto-Protect ability as well. The Force Shield gives out a permanent Shell status, but only when equipped before battle.

Final Fantasy VIII
Auto-Shell is learned on the GF Doomtrain for 250 AP. It can also be taught to any GF by the item Moon Curtain. Moon Curtain can be refined from 20 Magic Armlets or 100 Malboro Tentacles with Eden's GFAbl Med-RF ability.

Final Fantasy X
Auto-Shell is an auto ability. It can be found on dropped armor or customized onto armor by using 80 Lunar Curtain items. Characters with the equipped armor will permanently have the Shell status. It cannot be removed with Dispel.

Final Fantasy X-2
Auto-Shell is an ability found on the Mascot dressphere, and requires 80 AP to learn. The ability can also be accessed via the accessory Moon Bracer and by passing through the red gate on the Selene Guard Garment Grid.

Final Fantasy XII
The Shell Shield bestows an automatic Shell effect. It is a hunt reward for the Wyvern Lord and can also be found in the secret passage in Stilshrine of Miriam. In the Ward of the Sword-King there is a junction: the path to the left has a wall that will vanish if the player approaches it in a way the camera will attempt to go "inside" the wall. Shell Shield can also be found in a randomly spawning treasure in Cerobi Steppe and Tchita Uplands' Highlands, but the chance of it spawning is low.

Final Fantasy XIII
The Soulfront Talisman bestows Auto-Shell, but, unlike other incarnations, it will naturally wear off after a certain amount of time in a battle. The same effect can be gained by using a Aegisol.

Final Fantasy Tactics
The Ragnarok and Durandal knight swords impart the Auto-Shell ability when equipped. The effect can also be gained by equipping the accessories Sortilége and Tynar Rouge.