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The Defense stat affects the amount of damage sustained from attacks and in turn determines how much damage is taken away from the character's HP bar. Choosing the Dream Shield in the opening events of the first and second game will increase the character's overall defense. Defense can be raised using the Defense Boost (or Defense Up) item, equipping certain accessories and as the character progresses up the level table.
Defense follows similar mechanics as Strength and are both primary factors of successive battle, so having high values in both areas can denote a good balance and also success in battle.
Different Users for Defense
Terra, Ventus, and Sora
These three use their defense the same way. They jolt their arm up and use the Keyblade to help block. Terra's animation for the block is a jolt up, Ventus's is a flip around, and Sora's differs in all games in the series, but it is every time a different animation of jolts.
Aqua and Riku
Aqua and Riku both use different defenses from the rest, because they both use energy shields. Riku's can only go in front of him, while Aqua's can go all around, which is similar to Sora's Reflect Command in Kingdom Hearts II.