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The Critical Hit function is a battle element that has been present since the first Kingdom Hearts installment. It is always at the end of the combo. It adds extra damage to the finisher, and can also send enemies flying or stun them. Notably, Kingdom Hearts 358/2 Days and Kingdom Hearts Re:Coded are the only installments so far that have a visible stat of Critical Hit. In Kingdom Hearts 358/2 Days, it is possible for a Critical Hit to be dealt during Limit Breaks at any time. All the other games use a hidden value.
Critical Hits are extremely important for defeating bosses and particular Heartless. Its importance can be backed by how defeating a Black Fungus with a Critical Hit is the only way to obtain the rare Mystery Goo.
When in Atlantica, the easiest way to defeat a Sheltering Zone Heartless is to defeat it right on a Critical Hit. This stops it from exploding into more Sea Neons. It's also a factor that should be taken into consideration when choosing a Keychain, as many Keychains offer critical hit bonuses.
In Kingdom Hearts, the Critical Hit chances and damage can be adjusted by Keychains and Items. The critical hit damage is an addition onto the damage dealt by the finisher, so by equipping a finisher (such as Blitz), when a Critical Hit is dealt, the damage would be greater than if you had no finish ability equipped. When using activated abilities such as Ars Arcanum, the critical hit can only be obtained on specific hits. Strike Raid and Sonic Blade can only get a Critical Hit in the last hit, the 7th and 13th hit of Ars Arcanum, the final hit of the air combo segment of Ragnarok, and the blast from Trinity Limit.
In Kingdom Hearts II, the critical hit function has been merged with the combo finisher. The damage can be increased by the ability Combo Boost, as the ability increases the power of the finisher relative to the amount of hits the combo contains (increased by equipping multiple Combo Plus). It's unknown how much damage this adds onto the Critical Hit, but it will always be more powerful than a Critical Hit dealt without the Combo Boost ability. In addition to this, if a finisher ability such as Explosion is used, then the Critical Hit damage is increased. It could be increased significantly by having a combination of Combo Boost, Combo Plus and a strong finisher. The best way to maximize damage through Critical Hits is to equip a combonation of Combo Boost, Combo Plus, and Finisher abilities.
Different cards in Kingdom Hearts: Chain of Memories have different critical hit chances, while in Kingdom Hearts 358/2 Days, the Critical Hit is a visible stat that is separated into "Critical Hit %" and "Critical Hit Damage". Critical Hit % raises the chances of you dealing a Critical Hit, and the Critical Hit Damage raises the additional damage that will be dealt. The chance and damage can be raised even more by Rings such as the Critical Ring, and each Gear also has its own base stat of Critical Hit % and Critical Hit Damage.
Kingdom Hearts Birth by Sleep's Critical Hits are changed by each different Keychain. There are no additional ways to modify the Critical Hit value.
In Kingdom Hearts Re:coded, Critical Hit chances can be increased through accessories (Strike Armlet) and Keyblade overclock abilities (Oblivion ~ All Critical).