Guard and Protect are a set of Support-class Psynergy aligned with the Mars element featured in both games. Its primary usage is to bolster the defense levels of party members. It is available to the Guard Class series, the Mars Adept variation of the Swordsman Class series (accessible to Mars Adepts and also to Mercury Adepts to a limited degree), and the Page Class series (Enchanter onwards) accessible to Mars Adepts and also to Jupiter Adepts to a limited degree. Hence, six of the eight Adepts can access this series of Psynergy, with only the Venus Adepts being left out.
The Guard Psynergy series, in function, visuals, and statistics in general, is completely identical to the Guardian Psynergy series held by the Samurai Class series, so they might as well be treated as the same Psynergy. Since the Venus Adepts Isaac and Felix can become Samurai, it is as if the Guard Psynergy series is accessible by everyone.
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.
A fairly staple move within the RPG genre, raising your own defence can be a valuable tactic. However, as most bosses can cast the Break Psynergy, and it only lasts for a short period of time, it is quite possible to have the effect wear off before the enemy attacks the buffed adept, effectively wasting a turn and PP. The defence 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.