Chain of Memories is quite different from other games in the Kingdom Hearts series due mostly to its card-based battle system. Instead of having a list of commands, the player has a card reel. The selected card (like the selected command) is used as the attack. Unlike commands, cards have to recharge. Playing as Sora in Sora's Story differs from playing as Riku in Reverse Rebirth. Though it's not too much to adapt to after playing the another's story path, there are some major changes (Riku can't use any Magic cards, Sora doesn't have Dark Points, Riku doesn't have any friend cards other than King Mickey, Sora can't switch into a more powerful form during battle, etc.).
When you see Heartless roaming around the area, it's a sign that it's battle time. Like Final Fantasy, battles are entered when the enemies are walked into. This is unlike other Kingdom Hearts games, where battles were entered simply when the player character would walk in range of a Heartless. Walking into one Heartless initiates a battle with many Heartless. Sometimes cards are earned after finishing battles, like Map Cards and Enemy Cards. Map Cards allow you to go to other rooms on the floor's map. Additionally, battles are the best way to earn experience, allowing you to level up and face tougher bosses. The screen setup is as follows:
- Sora's/Riku’s HP bar is at the top-left corner of the screen. Additionally, Riku's Dark Points are also in that corner of the screen, and is HP bar is set up like the enemy's rather than Sora's (though his portrait is still present).
- Your deck will be in the bottom-left corner. You'll go through your card reel when you use the card at the top of the deck. When a black card with a number is on top of your deck, you probably ran out of cards. Hold while it's at the top of your deck and it will reload, allowing you to use it again.
- The cards your opponent is using will be in the bottom-right corner. You can also see when your opponent is charging their cards, so you know when to attack.
- Your opponent's HP will be in the form of a horizontal bar (like Riku's HP) at the top-right corner of the screen. Get it depleted to defeat your opponent.
Cards are the most vital part of the battle system of Chain of Memories. Form decks in rooms when you're not in a battle by using the menu. The more cards you have, the less you'll have to reload your deck during battle. There are four types of basic cards that can be used:
- Attack Cards are identified by a red border and an image of a Keyblade. Sora can switch attack cards and use many different types of Keyblades, while Riku is limited to one - the Soul Eater.
- Magic Cards are identified by a blue border and an image of an element. These cards can be used exclusively by Sora, and can be combined to make more powerful magic sleights like Firaga.
- Item Cards are identified by a green border and an image of an item. These cards can be used by both Sora and Riku. They act as items would in any other Kingdom Hearts game.
- Summon Cards are identified, like Magic Cards, by a blue border and the portrait of a friendly character. These cards can be used exclusively by Sora. When these cards are used in battle, Sora will jump out of the battlefield and the summoned character will jump in and perform an attack.
Though Attack Cards, Magic Cards, Summon Cards, and Item Cards are the main cards to use in battle, other useful cards can be obtained as well. These card types are:
- Friend Cards are, like Item Cards and Gimmick Cards, identified by a green border and the portrait of a friendly character. These are only obtained when allies join you in their respective worlds (Aladdin in Agrabah, Jack Skellington in Halloween Town, etc.). However, these only apply for Sora. The only Friend Card Riku can use is the Mickey one.
- Enemy Cards are identified by a black border and the portrait of an enemy. They can be used to boost the effect of attacks and other various reasons, and can be used by both Sora and Riku. They can usually be found when defeating enemies or bosses.
- Gimmick Cards are identified by a green border, like Friend Cards and Item Cards, and a Mickey Mouse insignia. They can be used by both Sora and Riku. The value of them is always 0, and they usually either change the battle area to let you gain the upper hand or stun the boss.
Card values are usually displayed by the number in the bottom-left corner of the card. These are important when trying to break the enemy's cards. If the number on your card is higher, you'll be able to break your enemy's card and your attack will follow-through. Press and when a card is on top of your deck to stock it. Stock three cards to make a Sleight. Sleights are combinations of cards that increase the power of the original cards.