Critical Hit
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 is the only installment so far that has a visible stat of Critical Hit. The other games use a hidden value. It's possible for a Critical Hit to be dealt during Limits at any time.
Importance
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 Mold.
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.
Differences
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, when a Critical Hit is dealt, the damage would be greater than if you had no finish ability equipped. When using Limits, 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 seventh and thirteenth hits of Ars Arcanum, the final hit of the air combo segment of Ragnarok, and the blast from Trinity Limit. The abilities Counterattack and Slapshot also can be Critical Hits, although in the former's case, Critical Hits are always activated.
In Kingdom Hearts II, The Critical Hit function has been absorbed into the Finisher of the combo all together. The damage can be increased by the ability Combo Boost, as the ability increases the finisher relative to the amount of hits the combo contains before it is finished (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 such as Explosion is used, then the finisher will also be stronger increasing the Critical Hit's power. It could be increased significantly by having a combination of Combo Boost, Combo Plus and a strong finisher.
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 has its own base stat of Critical Hit % and Critical Hit Damage.
In Kingdom Hearts 358/2 Days, different Keyblades provide different Critical Rate and the damage done by it. By linking Sight Unit Ⓛ, one can increase both Critical Rate and Critical Bonus. Some accessories can also increase them, such as Critical Sun.
The chances of Critical Hits occurring in Kingdom Hearts Birth by Sleep can be changed by the Keychain the player has equipped.
In Kingdom Hearts Re:coded, Critical Hit rate cannot be adjusted at all. However, the All Critical overclock ability can cause all basic attacks and attack commands to deal critical damage.
