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. This may be through love, power, or other unknown virtues. It is thought that only a person with a strong heart can wield a Keyblade.

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.

Sora
Sora's Keyblade was literally given to him when his home world, Destiny Islands was destroyed by the darkness. Sora's Keyblade is a Keyblade from the Realm of Light. The keyblade had originally chosen Riku as its wielder, but when Riku chose to follow the darkness in his heart, he passed on the Keyblade to Sora. Its default key chain is the Kingdom Key. 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 Sora use it, and instantly returns to his hand when taken by another. Sora is also able to summon it back to him, as seen during his fight against Roxas. The Keyblade did return to Riku for a short time, as he was the intended wielder, but it returned to Sora after Sora had shown that his heart was stronger than Riku's.

King Mickey
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.The replica model litter 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. Thus, it has been hinted that Mickey possessed it while under Yen Sid, which is how he met Ansem the Wise.

Riku
Riku was the true wielder of the Keyblade 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. 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
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. 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.

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.