Guard is a single-target Psynergy, that can be targeted on any currently-battling non-downed member of the party. Protect functions similarly, but affects all non-downed actively battling party members.
Guard: “Boost ally's Defense.”: This costs 3 Psynergy Points and affects one party member. Red-white energy gathers into the Adept and a shield image appears above the Adept to signify enhanced defense. The Adept's defense rating is temporarily improved by 25%; in Golden Sun games, a party member's defense can be enhanced up to a maximum of 150% of his or her normal defense rating, so using this twice will cause that Adept to attain 150% defense. It is available at level 3.
Protect: “Boost party's Defense.”: This costs 5 Psynergy Points and affects all party members. Red-white energy gathers into all party members and a shield image appears above each party member to signify enhanced defense. All Adepts' defense ratings are improved by 12.5%, so using this four times will cause all Adepts to attain 150% of their normal defense levels temporarily during battle. It is available at level 15.
Visually, Guard features red spheres of energy gathering into the selected target. A shield then forms in front of the targeted ally, before quickly dissipating to signify an increase in defense. Protect is nearly identical, simply multiplying the effect to show each ally's Defense increasing.
In the Golden Sun series, the maximum amount that an individual's Defense can be raised is 50% above the standard Defense. Thus, Guard can be used a maximum of two times on one ally, while Protect requires 4 uses to raise the Defense to maximum. Since Guard and Protect increases Defense by a percentage, the numerical increase is larger for Adepts with higher Defenses.The effects of Guard and Protect are only temporary, however, repeated re-castings can keep the Defense-bonus from wearing off. Additionally, the effects of Break can also remove any bonuses from the use of Guard and/or Protect.
Guard and Protect are available to a wide variety of classes: the Guard class series, specifically the Warrior class and higher, the Swordsman class series, the Pirate class series and the Page class series, specifically the Enchanter stage and onwards. Thus it is available to Mars, Mercury, and Jupiter Adepts. Non-Mars Adepts typically need large numbers of Set Mars Djinn to access Guard and Protect. Guard is learned at Level 3 while Protect is learned at Level 15.
Raising Defense is a fairly staple and viable tactic within the RPG genre. Early on, when the player's party has weaker armor and statistics, an increase in Defense can be extremely useful. In Golden Sun and The Lost Age, however, most bosses can cast the Break Psynergy, and since the Defense bonus is temporary, it is quite possible to have the effect wear off before the enemy attacks the buffed adept, effectively wasting a turn and PP. The defense multiplying effect means this move becomes more valuable as the party progresses, as it buffs the party by a larger absolute value. This can make it useful against bosses that have high attack ratings, though it depends heavily on the situation. For example, Guard and Protect are especially useful against the Ogre Titans, which use only physical attacks. In this case, increase Defense via Guard/Protect allows the player to reduce damage over a long, drawn out battle.