Restore is a Mercury-based Support-class Psynergy which removes Stun, Sleep, Delusion, and Curse. Restore is Psynergy widely available; despite the fact that only Piers, Mia, Rief, and Amiti learn it by default. The Swordsman (Venus), Swordsman (Mars), Dragoon, Seer (Mercury), Pilgrim (Mercury), Tamer, and White Mage class series can all learn this Psynergy; this means that all Adepts can possess it.
Despite curing four separate status effects, Restore is not actually very useful. Stun will generally be cured naturally after one turn, so casting Restore to end the effect is rather meaningless, and would only serve to slow the party down more. Delusion is also not a particularly worrying status effect, as enemies that withstand enough attacks to make the player's missed attacks add up are generally going to be targeted with something stronger than standard normal attacks anyway. Sleep may be relevant, as it usually targets more than one Adept and lasts for a longer period than Stun. Curse may also need dispelling in some of the longer boss battles. However, in this case it is entirely probable that the Adept(s) with Restore is rendered unable to cast the Psynergy until the status effects wear off naturally, which decreases the value further. It also does not dispel Haunt, which otherwise would have made it invaluable. Finally, in major battles many enemies attack multiple times, so it is highly likely that Restoring the Adept(s) will simply lead to them getting re-afflicted immediately afterwards. Thus it is often simpler to just ignore the status effects and try to defeat the enemy, as all of these impediments will be removed at the end of the battle anyway.