This is the walkthrough for Dive to the Heart from Kingdom Hearts. While it follows the content from the Final Mix version, it is largely applicable to the original release as well. The main gameplay differences are the changes in order of abilities gained upon leveling up, several new and powerful abilities, a harder difficulty mode, several treasure chest contents, as well as new Heartless, weapons, and accessories.
After selecting a difficulty level, other settings, and watching the intro scenes, you will gain control of Sora. The mysterious voice will urge you to move him forward, which triggers another cutscene, after which you must make a choice between three items, the Dream Sword, Dream Rod, and the Dream Shield. You must choose one, and sacrifice another, which will affect your starting attributes and the order in which you receive new abilities when gaining levels. The Strength, Defense, and Ability Points statistics matter little, as they can easily be balanced out with items. However, the Rod is the only one that grants an extra two Magic Points if picked, which is notable as max MP affects your magic damage. On the other hand, the Sword and Shield give an additional one and two usable item slots, respectively, which can be useful if you need to rely on extra sources of healing.
The more important factor is the order of the abilities gained upon leveling (which is also different if you are playing the original version). In general all three routes are viable, but picking the Shield grants the best defensive abilities relatively early. Comparatively Sword and Rod focus more on their own offensive and passive abilities, but are otherwise relatively well balanced out. Ultimately each choice leads to the same abilities at level 100.
After another cutscene you will be prompted to perform an attack, after which you unlock the bars for your Hit Points and Magic Points, followed by the spawning of several Shadow Heartless. These often attack in groups, and can meld into the ground, becoming invincible, but will still able to move around. They otherwise use short range claw swipes, lunges, and jump attacks. Kill one to force the rest to retreat, then pick up the HP orbs while the on-screen messages explains Experience and items. Another group will appear afterwards, and you will also gain the use of the lock-on functionality, which will help in defeating the group of Shadows as they rely on their mobility and numbers to overwhelm you. Note that you cannot actually die at any point for now, as you get automatically healed if your HP gets too low.
Make sure to fully finish any of your combos if possible, as the third hit is the strongest, and has a wider arc than the rest, often hitting multiple enemies. After another cutscene you will end up on a new platform with a transparent door. The game will now explain how to interact with objects, so examine the door, then open the treasure chest, followed by pushing the large crate, finishing it by destroying it, which results in a free Potion. You can, and should, put usable items in your inventory so you can heal during combat. Lastly, destroy the barrel to make the door tangible, then open it.
This time you will have to talk to three characters; Wakka, Tidus, and Selphie. They each ask a question, with only three available answers. The actual contents are irrelevant, but the number of the answer chosen affects your Experience gain. Picking the first answers at least twice (Dawn) will make it 20% faster up to level forty, after which it slows down, picking the third answers at least twice (Dusk) will make it 30% slower up to level forty, after which it speeds up. Additionally, the latter option (Dusk) requires significantly less EXP to reach level hundred. Midday, the option that is chosen if you pick any other equal combination of answers, is the most balanced out of the three.
Afterwards the location returns to another platform, where you will be ambushed by a large group of Shadows. Make sure to pick up any dropped health orbs to restore HP, saving your Potion for later. After their defeat a save point will appear, allowing you to restore your HP as well as save the game. Once done, follow the new path to another tower to enter a bossfight.
The Darkside is large, but will not move from its position, and its only vulnerable points are its hands and head. It will begin by smashing into the ground with its fist, which is easily dodged, summoning several Shadows in the process. The Shadows are a nuisance, but can provide some valuable health orbs. If you can, try to jump up its arm and reach its shoulder to start hitting its head for some extra damage and free Tech Points (bonus EXP), otherwise resort to hitting the fist.
Get rid of any Shadows before it uses its next attack, in which it goes down onto its knees, splays its arms behind it, and propels several homing orbs from the hole in its chest. The orbs can be struck and deflected back, but otherwise staying behind it (where they are slightly unlikely to reach you) and using the opportunity to hit one of the hands is more effective. It will cycle between these two moves, so just repeat your actions until it finally dies, using your Potion if you need it. Alternatively, if you do die you will not gain a game over, but instead only lose out on some valuable early experience points.