Ply Psynergy series

Ply, Ply Well and Pure Ply are a series of Mercury Recovery-class Psynergy featured in both versions of Golden Sun. It is available to the following classes:
 * Swordsman (Venus)
 * Swordsman (Mars)
 * Dragoon
 * Water Seer
 * Mariner

Psynergy Spells
Ply: "Restore 100 HP with faith's power.": This spell costs 5 Psynergy Points, restores 100 HP, and affects one target ally. It is available at Level 1 for Swordsman (Venus), Swordsman (Mars), Dragoon, and Water Seer, and at Level 2 for Mariner.

Ply Well: "Restore 200 HP with faith's power.": This spell costs 8 Psynergy points, restores 200 HP, and affects one target ally. It is available at Level 16 for Swordsman (Venus), Swordsman (Mars), Dragoon, and Water Seer, and at Level 17 for Mariner.

Pure Ply: "Restore 1,000 HP with faith's power." This spell costs 12 Psynergy Points, restores 1,000 HP, and affects one target ally. It is available at Level 34 for Swordsman (Venus), Swordsman (Mars), Dragoon, and Water Seer, and at Level 35 for Mariner.

Analysis

 * This spell is the single-most potent Recovery Psynergy in the game, short of reviving a fallen ally. As for whether it's the most practical, most would award that honor to Pure Wish.

Ply as a Utility Psynergy
Ply can be used outside of battle, usually to heal injured party members. However, Ply has another purpose: it can (and must) be used to activate certain events in Mercury Lighthouse: When Ply is used in this manner, a fairy appears and sprinkles some "fairy dust" in the area, after which one of the mentioned effects will activate. This effect of Ply can only be used in the above locations or by setting it to a shortcut.
 * Ply is used open the front door of Mercury Lighthouse (Mia does this automatically before joining the party).
 * It is also used on a statue near the entrance (although inaccessible except from above) to activate special panels. These panels allow Isaac's party to jump across the water for a limited time.
 * Finally, it is used on a waterfall near the end of the dungeon to allow Isaac's team access to the Aerie.

Ply as a Monster's Psynergy
Another form of Ply is seen used by the Refresh Ball in the Star Magician battle. The ability is called Earnest Ply and appears to have a limited effect similar to Pure Ply. However, it may only target the Star Magician. When hacked onto a character, the spell only allows to be cast on one's self. Other than the limited target selection, though, it is identical in every way - from amount healed, to PP cost.