Critical Hit
Critical Hit is a battle feature introduced in Kingdom Hearts. It is a finishing blow always present at the end of a combo and also during some magic abilities and possesses more damage power than the follow-up combination attacks. The magic ability Trinity Limit is rich in critical hits.
Furthermore, keychains and Sora's ability Critical Plus can help the chances of critical hits being present in a combination and also determine the strength of the critical hit. Examples of keychains that encourage critical hits are Fairy Harp, Pumpkinhead, Olympia Metal Chocobo, and especially Wishing Star. Depending on the Keychain's Passive Ability, the strength of the critical hit and its overall presence in the combo varies.
In Kingdom Hearts 358/2 Days, equipping a character with the Strike Ring, Lunar Strike or Critical Sun rings will increase Critical Hit Ratio, and equipping a weapon with Sight Units will also increase Critical Hit Ratio. The critcal hit is indicated when yellow lines appear from the attack and the attack has a change to its sound.
Importance
Critical Hits are important, since they help with defeating bosses because they deal more damage. Also if you are after the rarest item in Kingdom Hearts, Mystery Mold, then you must defeat a Black Fungus right on a critical hit. It is best done using the Trinity Limit magic ability.
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.
Differences
The way critical hits affect gameplay varies across the different games in the Kingdom Hearts series. Generally, critical hits have a higher damage multiplier than the standard hits in a combo (meaning that the players current strength value is multiplied by a another value to take off more health from the enemy).
In the original Kingdom Hearts, critical hits consist of the final hit in a ground or air combo, the final hit in Strike Raid and Sonic Blade, the 7th and 13th hits in Ars Arcanum, the final hit in the short air combo of Ragnarok, and the blast from Trinity Limit. There are possibly other attacks that are considered critical hits due to an increased damage multiplier, however the Kingdom Hearts Official Strategy Guide does not go into as much detail on this as the Kingdom Hearts II Official Strategy Guide does.
Through the use of certain abilities it is possible to further increase the power of critical hits. If the Critical Plus ability is equipped, or if the player is using a weapon with that passive ability, the character will have a chance of inflicting more damage with a critical hit. There are also some action abilities that can be equipped to have a similar effect. Blitz is an ability Sora can use to replace his finishing ground hit with a more powerful attack resulting in a more powerful critical hit.
In Kingdom Hearts II, critical hits function similarly, but are a more important game element. All bosses in the game will not drop below 1 HP unless they are defeated with a critical hit. The Kingdom Hearts II Official Strategy Guide lists all characters attacks, magic, and action abilities and beside them states weather they deal critical or "Final Blows". The Critical Plus ability is also present in Kingdom Hearts II (although it is referred to as Combo Boost), but it functions differently than it's predecessor. Equipping Combo Boost increases the damage multiplier of the final hit in a combo relative to the number of standard hits in that combo. This works by simply increasing the value of the power multiplier for the final move. The exact rate at which combo boost increases the damage of the critical hit is unknown but the damage the final hit does to enemies is noticeably larger with it equipped.
