The battles of the Kingdom Hearts series are based off of a system of Health Points.
Each character and enemy has a number of Health Points that must be depleted before he/she/it is defeated. To deplete Health Points, that character or enemy must be hit by an attack or spell that causes damage. The amount of Health Points taken away with each hit depends on the attacker's strength and the target's defense. A higher defense means that less points are taken away with each hit, and higher strength means that more points are depleted.
Allied characters' health can be seen next to their picture on the Heads Up Display, and enemies' health can be seen on the top of the screen when targeted.
Allied characters' health can be restored via HP orbs, Potions, Hi-Potions, Mega Potions, Elixirs, and Cure Magic. Some enemies have the ability to regenerate their own health, but this is not common.
Health points are displayed in a bar form. When a character's health is at 100%, the bar is completely green. when damage is taken, part of the green becomes black, signifying that there is not full health. When health reaches a very low point, sora goes into critical mode. There are no benefits to this, however the Health meter will flach and a siren will go off. This is to warn the player that they are low on health.
In Kingdom Hearts:Chain of Memories, there is a second health bar that is blue. The blue health meter must be depleted before the green meter begins to run out. This second bar is based off that of bosses in that game, in which the second HP meter was blue.
Enemy health has several appearances. In Kingdom Hearts, the standard meter was green. a second bar was yellow, then red, followed by pink and purple. If an enemy had more health than could be displayed in 5 meters, then it would appear as if no damage was occurring, until the amount of health could be displayed in the 5th bar.
In Kingdom Hearts:Chain of Memories, health was displayed in a similar way. The first meter was green. On top of that could be blue. Above that was yellow, and then red. In Kingdom Hearts II, the style was different completely. All meters were green, and additional mevers were represented by green squares underneath the Health bar
Increasing Max Health Points
In each game, the way in which one can increase Sora's max health differs. In Kingdom Hearts, additional Health was automatically added upon a level up. In Kingdom Hearts: Chain of Memories, increasing max health was an option upon level up, along with increasing the deck size and learning a new Sleight. In Kingdom Hearts II]], max health is increased as a reward for completing certain tasks, usually defeatting a Boss.