The battles in the Kingdom Hearts series are based upon a system of Hit Points (HP).
Each character and enemy has a number of Hit Points that must be depleted before he/she/it is defeated. To deplete Hit Points, that character or enemy must be hit by an attack or spell that causes damage. The amount of Hit Points taken away with each hit depends on the attacker's strength and the target's defense. A higher defense means that fewer points are taken away with each hit, and higher strength means that more points are depleted.
Allied characters' HP can be seen next to their picture on the Heads Up Display, and, if the player has the Scan ability equipped, the enemy's HP can be seen on the top of the screen when targeted.
Allied characters' HP can be restored via HP orbs, Potions, Hi-Potions, Mega Potions, Elixirs, Megalixirs, and Cure Magic. Some enemies have the ability to regenerate their own HP, but this is not common.
Hit Points are displayed in a bar form. When a character's HP is at 100%, the bar is completely green. When damage is taken, the picture of Sora on the meter flinches momentarily and part of the green becomes black, signifying that there is not full health. When HP reaches a very low point, Sora goes into critical mode. The HP meter will then flash red and a siren will go off. This is to warn the player that Sora is low on health. This carries on into Kingdom Hearts: Chain of Memories and Kingdom Hearts II and also applies to both Roxas and Riku when they are playable. In Kingdom Hearts 358/2 Days and Kingdom Hearts Birth by Sleep, the character portraits do not flinch, but merely flash red when taking damage. The HP bar also flashes yellow when picking up Mission Point prizes in Kingdom Hearts 358/2 Days. As in past games, an alarm still sounds when low on health.
The HP bar is often located to the bottom-right or top-left corner of the screen, grouped with all other gauges displayed during gameplay. In Kingdom Hearts II, the HP bar appears next to the MP and Drive Gauges. It only appears next to the MP Gauge in Kingdom Hearts, and in Kingdom Hearts: Chain of Memories, it appears next to Riku's DP Gauge in Reverse/Rebirth. In Kingdom Hearts Birth by Sleep, it appears next to the Focus and D-Link Gauges. In Kingdom Hearts 358/2 Days, no other gauges appear, but the Limit Gauge serves as, at most, half of the player's HP bar.
In Kingdom Hearts: Chain of Memories, there is a second HP bar that is blue. The blue HP 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 HP bars have several appearances. In Kingdom Hearts, the standard meter was green. A second bar was yellow, then orange, followed by red and purple. If an enemy had more than 1,500HP, 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, HP 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, Kingdom Hearts Re:Chain of Memories , Kingdom Hearts coded, and Kingdom Hearts Birth by Sleep , the style was completely different. All meters were green, and additional meters were represented by green squares underneath the HP bar. The same thing was done in Kingdom Hearts 358/2 Days, except little ovals were used instead of squares to represent multiple HP bars.
 Increasing Max Hit Points
In each game, the way in which one can increase Sora's max HP differs. In Kingdom Hearts, additional HP was automatically added upon a level up. In Kingdom Hearts: Chain of Memories, increasing max HP was an option upon level up, along with increasing the deck size and learning a new Sleight when playing as Sora, and increase Attack Points and Dark Points when playing as Riku. In Kingdom Hearts II and Kingdom Hearts Birth By Sleep, max HP is increased as a reward for completing certain tasks, usually defeating a Boss. The Maximum Hit Points in Kingdom Hearts Re:Chain of Memories is 560.
In Kingdom Hearts and, the overall number of Hit Points obtained throughout the game depends on what weapon the players chooses at the start. The most Hit Points can be obtained if the player chooses the Dream Shield, this enables Sora to reach a maximum number of 111 HP by the end of the game. Likewise each of the three Dream Weapons, correspond with either Strength, Magic, Defense or HP.
In Kingdom Hearts II, the maximum number of Hit Points for Sora is 120 regardless of what weapon the player chooses at the start of the game.
In Kingdom Hearts 358/2 Days Story Mode, Roxas begins with 62 max HP and each of his levels increases the max HP by 3. In Mission Mode, the max HP differs around characters. Lexaeus has the highest max HP status and Zexion and Donald both have the lowest. Max HP can only be increased by leveling up or installing a ring panel that increases max HP.