Keyblade

The Keyblades are mysterious weapons that partially have a will of their own. It appears that there was once a "Keyblade War" in which Keyblades were common weapons. Many Keyblades can be seen in the secret ending of Kingdom Hearts II Final Mix+, although none of these nor those of Aqua, Terra or Ven, have chains. The legend of the Keyblade warrior appears to contradict itself. One legend states that the Keyblade master saved the world, while another said he wrought chaos and destruction.

Acquisition
A Keyblade is said to choose its own master. It is known that only one with a strong heart can wield a keyblade, but exactly what this entails is left to interpretation(powerful, virtuous, etc.).

Theories to the keyblade
There are many theories that speculate on the purpose of the Keyblade, as well as why some Keyblades do not have a keychain and are therefore called fakes. Various complications occur when the crossroad with Keyblades with no keychain from previous games were shown. While the origins of the keychains are misty, there are many theories to it's purpose. Such as that they are charms that summon the keyblade to it's host. Another theory speculates that when one creates a keyblade, as Xehanort did by the use of Riku, there is no keychain with it. That would indicate that the keyblade can then be created either by the person or from the person, as Xehanort's Keyblade was created from the Princesses of Heart, or how Riku's Keyblade was created from the Soul Eater, which was forged out of the darkness in Riku. The keyblade can then be seen as an extension of it's creator, and that a Keyblade's power comes from its creator alone. Therefore, when a keyblade needs to be passed to a person, a keychain is formed to be a power source to give the keyblade lasting energy as well as its power and form. This could possibly explain why some keyblades do not have keychains such as Terra, Ven and Aqua's Keyblade. It could also show how Riku got his Way to the Dawn Keyblade as well. This would mean each keyblade has its own unique power and form if keyblades were created by certain individuals, because people are unique and the same couldn't be said for Keyblades.

Powers
As the name implies, in combat, Keyblades are used in a manner similar to swords, and have great cutting power despite not usually having an edge. They can also be used as conduits for magic skills such as Fire or Blizzard. Keyblades are also one of the few weapons in existence capable of harming Heartless and Nobodies.

The main power of the Keyblade, however, is the ability to lock (and possibly unlock) the Keyholes of worlds, sealing away its heart. Possibly as an offset of this ability, the Keyblade can also be used to lock or unlock any lock, allowing the user to access any sealed interior, whether it be a locked room, gate or treasure chest.

In addition to its base powers, Sora's Keyblade in particular has been shown to be customizable via keychains he finds or receives along his journey, giving it additional abilities or boosting its offensive and/or magical powers. Furthermore, the Keyblade's appearance changes depending on what keychain is attached. It is unknown of any other Keyblade besides Sora's can utilize keychains, though King Mickey's and Riku's keyblades do have keychains, so they MAY be able to, this is just a theory.

Keyblades
The Kingdom Key is the Keyblade of Sora. Sora's Keyblade was given to him in the dying minutes of his home world, Destiny Islands when it was destroyed by the darkness. A Keyblade from the Realm of Light its default key chain is a small set of three silver circles that form the symbol most commonly associated with Mickey Mouse, in the form of his head. It is commonly referred to within the game as "the Keyblade". Sora's key can seal Keyholes. It is yet to be revealed if any other Keyblades have this power (although the King's Keyblade was used in tandem with Sora's to seal the Door to Darkness.) The Keyblade also refuses to let anyone but those of stronger heart use it, and instantly returns to that person's hand when taken by another. Sora is also able to summon it back to him, as seen during his fight against Roxas in The World That Never Was. 
 * Kingdom Key:

Riku was the true wielder of the Kingdom Key of the Realm of Light, but when he chose to follow the darkness in his heart, he passed on the keyblade to Sora. He came to wield another keyblade during the course of Kingdom Hearts, while possessed by Xehanort's Heartless. The keyblade is refered to as the "Dark Keyblade" (also called "Artificial" as it has no official name, and rarely referred to as the 'Heartbreaker' to give it a more formidable name). It was made from six of the hearts of the Princesses of Heart and had the power to unlock peoples hearts. Riku first used it to unleash Maleficient's dragon form, then wielded it in combat against Sora, where he then lost it after the battle.

Riku's Keyblade in Kingdom Hearts II is the Way to the Dawn. It seems to be an evolved Soul Eater, which was formed from the darkness in his heart, but not much is clear about this Keyblade. Some speculate that the Keyblade could have been used as a median considering it's given appearance.

Kairi's currently-unnamed Keyblade was given to her by Riku in The World That Never Was. Kairi's Keyblade is delicate looking, and sports a flower design. Kairi used this Keyblade to fight (albeit inefficiently, despite Donald's praise) against the Heartless for one battle in The World That Never Was. The fact that Kairi is able to wield a Keyblade raises many questions that Nomura will most likely address in the future installments of the series, but it is likely because of how strong her heart was as shown in the first game as she had no heartless, a feat sora has not achieved. Tetsuya Nomura has said the scene in which Riku gives her a Keyblade hints at something. Some assume it's the keyblade that represents Destiny Island, given it's floral design.
 * Kairi's Keyblade:

King Mickey’s Keyblade is a Keyblade from the Realm of Darkness, which the King recieved sometime during his journey in that realm. It is identical to Sora's Keyblade, except the colors are reversed, the hilt being silver and the blade gold, this is also noted by fans, leading some to believe it is called The Kingblade or the Inverse Kingdom Key. The replica model letter opener called Mickey's Keyblade Darkside. Mickey used this Keyblade along with Sora's to seal the Door to Darkness, so it theoritically has the same powers as the its Realm of Light counterpart. Prior to that, Mickey originally possessed the Star Seeker, which is seen in the Kingdom Hearts II: Final Mix+ trailer for the upcoming PlayStation Portable game Birth By Sleep. Thus, it has been hinted that Mickey possessed it while under Yen Sid, which is how he met Ansem the Wise.
 * Mickey's Keyblade:

Keyblade Wielders

 * Sora
 * Kairi
 * Riku
 * King Mickey

Roxas
Roxas, being Sora's Nobody, also has the ability to use the Keyblade. This has earned him the name "Key of Destiny" from his fellow Organization members. The keyblade that Roxas used and the thing that Sora once lost in Castle Oblivion are the same thing. Furthermore, these two both used the keyblade at the same time. In order to learn why he could wield the keyblades, he betrayed Organization XIII and left in search of his true Self(Sora). Tetsuya Nomura stated that Roxas' ability to use two Keyblades means that Sora can, too, without the Drive Forms, of course. Due to the fact that Roxas has every ability that Sora has. Tetsuya Nomura said that the keyblades represnt Riku and Kairi, Riku being Oblivion, and Kairi being Oathkeeper. These all together (when Roxas fuses with Sora) completes Sora.

Terra
The Weapon that Terra uses throughout most of his appearances. He battles Sora with it in Kingdom Hearts II: Final Mix. Although it seems to be very powerful, he also takes the Kingdom Key in Kingdom Hearts II's secret movie. Because it lacks a chain, it is rumored to be a "false" Keyblade (however, this may only be speculation), and that seems to be the reason for him discarding it in the movie. However, the previous sequence described was merely concept and is not confirmed as an accurate interperetation of the event. However, when Master Xehanort appears, Terra quickly grabs his own weapon again, so he must have some sort of attachment to it. Ven and Aqua also seem to have weapons like this one.

Terra's keyblade displays powers that none of the others can do, in that it can transform into different weapons (either that or Terra has the power to transform it). It can change into a whip, a jet-ski like object, a fast spinning cog-like weapon, and a massive cannon.

Master Xehanort
It is confirmed that Master Xehanort wields a keyblade as well as his apprentice. What power his keyblade holds is shrouded in mystery.

Master Xehanort's Apprentice
Although Master Xehanort's Apprentice's keyblade is also shrouded in mystery, his keyblade takes a strong resemblance to Riku's Keyblade and hold great strength in thunder and ice attacks. He might have a connection to Riku.

Genie
The Genie from "Aladdin" also wields a keyblade. Although he takes nearly no part in the main story of the series, he wields a keyblade partly resembling the "3 wishes" keyblade. He obtains a keyblade by tapping on Sora's keyblade during a Summon in Kingdom Hearts II.

Xehanort's Heartless
Made from the Princesses of Heart.