Defend

Defend is a battling option in both Golden Sun games. When used, the defending Adept will do nothing for that turn (as defend has no animation), but it cuts the amount of damage taken from a foe by exactly 50% of what you'd get otherwise. Defend will offer this protection for the whole turn, and thus works irrespective of the user's (or enemy's) agility rating.

When a character is going to attack an enemy that is already downed they will automatically defend. This was actually the cause of a lot of negative remarks in the reviewer community which called it a lack of "smart combat" as said by an IGN reviewer. Defending does not reduce your chances of being inflicted with a status affecting attack. However, it does also cut in half any damage done to your PP meter. Several monsters in both games have ability to defend. For a full list of these enemies, see here.

Analysis
As a command, defend is almost always an undesirable option to use in battle, as a defending Adept cannot perform any other action and thus will not be able to aid the party on that turn. While the effect (reducing the rate of HP loss) is useful, defend is almost never the best choice, as increasing the rate of healing is almost always a much more effective tactic as healing items and Psynergy are plentiful and, with a few exceptions, will heal much more than 50% of an enemy's attack. However, for example, if the party cannot afford to loose any members and one or more Adepts do not have enough HP to survive a full hit, but the enemy acts first, then stalling until healing can be performed, by setting the endangered Adept(s) to defend, is potentially desirable. However, in this case, other defensive options (such as the Mars Djinni, Flash) are almost always better choices, as they offer greater protection and target more Adepts. Thus, the best time to use defend is when stalling is needed (it need not be waiting to restore HP, it could be Djinn recovery or status effects, such as Seal, etc), but all superior defensive options have already been consumed. Obviously, this is a niche situation that is both statistically and strategically unlikely to occur, and as such, it is far more likely that an Adept will simply be forced to use defend by default when an Adept's target in invalidated due to the monster being felled earlier in the turn.

If, however, the objective is to not win the fight as efficiently as possible, but rather to win it in a specific way, for example, via a method that utilises the Random Number Generator to ensure a 100% drop rate on rare items, then defend is a very handy option, as an inactive Adept does not add to the "Random Numbers", allowing a slower Adept to use an appropriate Djinni to fell the enemy and complete the sure-drop method.