If you do not know what Kingdom Hearts is, it is perhaps the most epic Disney game. Or one of the most epic, because now they are coming out with Disney Infinity and that game is just as epic as this game.
Anyway, in most Kingdom Hearts games, you play as Sora, a young hero with the ability to wield a weapon known as a Key-Blade. However in the game Kingdom Hearts: 358/2 Days, you play as Roxas, who happens to be a Nobody. No really, that's the name of his species. Nobodies are basically clones of normal people. However they are created when one becomes a Heartless, which is similar to a Nobody in the way that both literally do not have a heart. However, Nobodies are crafty, where Heartless act entirely on instinct.
Roxas can do everything Sora can because Roxas is the Nobody version of Sora. Because in the first Kingdom Hearts game, Sora released his own heart to save a very close friend named Kairi. Thus, he turned into a Heartless. However, Kairi somehow managed to restore Sora to his normal self. But the exact moment his heart left his body, Roxas was born.
In almost every game Sora fought side-by-side with two or more Disney characters, most notably Donald Duck (from Mickey Mouse) and Goofy (also from Mickey Mouse). Anyway, there are several worlds are based off of various Disney movies.
However, in Kingdom Hearts 3D: Dream Drop Distance, Neither Donald Duck or Goofy come with Sora. Because the whole game takes place in the Sleeping Worlds. And only people with a Key-Blade can go to the Sleeping Worlds. But only if the Key-Blade user is asleep (because it's the Sleeping Worlds). However, Sora does not go to the Sleeping Worlds alone. His best friend, Riku, also goes to the Sleeping Worlds. And here is why they must go to the Sleeping Worlds. One can "wake up" a Sleeping World by unlocking it's Key-Hole. All Worlds, including the Sleeping Worlds, have a Key-Hole. But you must find it before you can unlock it. If both Sora and Riku can awaken all seven Sleeping Worlds and return to reality they will become Key-Blade Masters. They both have Key-Blades, by the way.
But the Sleeping Worlds are just as dangerous as the normal worlds despite the fact that Neither Heartless or Nobodies cannot enter. Because the Sleeping Worlds have Dream Eaters. Some are good, others are evil. The Nightmares are evil, the Spirits are good. The only difference is that Nightmares have red eyes while Spirits have yellow eyes. Sora and Riku can team up with the Spirits to fight Nightmares. But how they use their Spirits is different. Sora summons and rides his Spirits like a horse rider riding a horse, While Riku absorbs the Spirit into his body to activate a brutal combo that cannot be done normally. Both Sora and Riku can also use Dual Links where they activate to of their Spirits at the exact same time to deal massive damage.
The game Kingdom Hearts: Birth by Sleep is another instance where the player does not use Sora. Instead they play as three other Key-Blade users named Ventus, Terra, and Aqua. They are each responsible for the origin of Sora's, Riku's, and Kairi's Key-Blade powers. You see, Ventus gave Sora the ability to use a Key-Blade because Ventus' heart is inside Sora's body. Riku became a Key-Blade user because Terra chose him as a successor. And Kairi unintentionally became Aqua's successor. Because if the Key-Blade accepts the successor, it will glow. And it glowed when she touched it. Ventus also appears as a cameo in 358/2 Days, because a special Nobody named Xion (all Nobodies have the letter "X" in their name) briefly turned into Ventus. Because unlike other Nobodies, Xion is literally made of memories, Sora's memories to be exact. Her appearance changes according to Sora's memories. That probably explains why she is normally identical to Kairi, because Sora's memories of Kairi are his strongest memories. Also in 358/2 Days, Sora was in a coma during almost the whole game, because in Kingdom Hearts: Chain of Memories, Sora lost most of his memories and while in a coma Sora's memories are slowly being restored and will wake up the moment his memories are completely restored.
Roxas also appears in other games. Sometimes he is a boss that must be defeated, other times he is a valuable ally. And remember Ventus? Good. Because their is a character known as Vanitas, and Vanitas kind of came out of Ventus' heart. In fact he is the physical form of the darkness within Ventus' heart. Later on in Birth by Sleep Vanitas and Ventus merge to create a combo of both. Ventus-Vanitas!!!! He has the same muscular build as Vanitas but the face of Ventus.
In Dream Drop Distance Sora and Riku had a new power called Flow-Motion. It allows Sora or Riku to move quickly at high speed and do awesome acrobatic feats. Like jump off walls, swinging on poles, and rail grinding. Another cool thing is that when you complete Sora's side of the story Ventus' battle armor appears to protect Sora from the darkness. Unfortunately, Ventus' armor was corrupted and turned into a Nightmare Dream Eater. That doubled as the final boss. So Riku defeated the Nightmare Armor and the game ends with Sora and Riku awakening. Unfortunately, only Riku became a Key-Blade Master. But Sora is not upset about it and congratulates Riku.
Kingdom Hearts was originally for PS2 and Nintendo DS (I do not have either). But now they came out with a Kingdom Hearts game for the PS3 (which I DO have)!!!!!! Well actually it's two Kingdom Hearts games (the original Kingdom Hearts game and Kingdom Hearts Re:Chain of Memories) and the cutscenes from 358/2 Days. Not actual gameplay, just the cutscenes. Two hours of cutscenes to be exact.
Sora, Riku, and Kairi each came from a place called Destiny Islands. It is somewhat ironic that Destiny Islands is Sora's, Riku's, and Kairi's homeworld because it is the very first world the Heartless attack (in fact they destroyed it and sent it to the End of the World).
Want to know why this video game series is called Kingdom Hearts? Because the in-game Kingdom Hearts is the heart of everything in the universe. In fact it's the heat OF the universe itself. The ultimate Light. Many Key-Blade users have tried to claim it, but that just resulted in the Key-Blade war. Now the only way to summon it is with the ultimate Key-Blade: the χ-Blade!!!!! Even though it only appeared in Birth by Sleep. Ansem, Seeker of Darkness (the leader of the Heartless) somehow found a way to Kingdom Hearts (without using the χ-Blade) and thought that the Kingdom Hearts would empower him with darkness, but all he got was light, and the light was so bright that it destroyed him in the blink of an eye.
For some reason, Sora (and other characters) would gain a new form to blend in if they go to certain worlds. For example, Sora would turn into a vampire if he is the Nightmare before Christmas world, or get Tron style armor if he goes to the Grid (A.K.A. the Tron world). Also, in all games, Sora (or Roxas, Aqua, Terra, Ventus, or Riku) can cast magic spells in the form of a long-ranged attack. Depending on the name of the spell that you want to cast, the attack can be different. Unless the spell contains the first syllable in a weaker spell's name, where the spell will be the same spell, just more powerful, and doing more damage. However, some spells (like Cura) are designed to restore lost health or remove negative status affects.
Also, Roxas is unique when compared to other Nobodies. You see, normal Nobodies don't really have emotions, but Roxas DOES have emotions. Which is probably why the Heartless attack him frequently because the Heartless are attracted to emotion, especially hate and anger. In fact one of the bosses in the first game (fought in the Tarzan world) is actually a hunter character that was kidnapped by Heartless and he, himself, became a Heartless. All because of his hatred and anger for any animal he has not killed. So Sora destroyed him after that hunter dude became a Heartless.
Another cool thing is in one game (I forgot which one), Sora, Donald Duck, Goofy, King Mickey (A.K.A. Mickey Mouse) and other characters had to fight in an event called the Battle of 1000 Heartless. True to the name, the player must destroy 1000 Heartless.
Here is a gallery. NOTE: ALL IMAGES ARE FROM THE KEY HOLE, THE KINGDOM HEARTS WIKI.
This is what Ventus', Terra's, and
Aqua's battle armor looks like.
This is Sora.
This is Riku.