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 2, although none of these 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. Rumor has it that only a person with a strong heart can wield a Keyblade.
Powers
A Keyblade can utilize magic abilities, such as Fire or Blizzard magic. A Keyblade can be summoned back to its master. Keyblades (or at least Sora's) can gain new powers and abilities through different keychains that can be attached to the blade. Keychains also change the appearance of the Keyblade. Keyblades also retain a magical sharpness, being able to cut through buildings cleanly in a single swipe, despite missing an edge. It also acts like a homing signal to the Heartless. Another feature is that it can lock or unlock any keyhole, not only Keyholes.
Kingdom Key
Sora's Keyblade was given to him when his home world, Destiny Islands was destroyed by the darkness. Sora's Keyblade is the Keyblade of the Realm of Light. 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. 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. This Keyblade's original chosen one was meant to be Riku, but after Riku gave in to the darkness during the attack on Destiny Islands, it chose Sora instead. The Keyblade did return to Riku for a short time, as he was the intended wielder, but it returned to Sora after Sora showed his heart was stronger than Riku's.
Mickey’s Keyblade
King Mickey’s Keyblade is the Keyblade of 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 switched, the hilt being silver and the blade gold, this is also noted by fans, leading some to believe it is called The Kingblade. It has been stated that this keyblade is called the "Darkside", since it was obtained when King Mickey entered the realm of Darkness. In the video clip, Darksides apperated, but were slain by Mickey, hence the name "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 KH2: FinalMix+ trailer. Thus it had been hinted that Mickey posessed it while under Yen Sid, which is how he met Ansem the Wise.
It is also believed that this keyblade was used by Aqua, one of the three knights during the Kingdom Hearts II and Final Mix+ trailers.
Riku's Keyblade
Riku's Keyblade is the Way to the Dawn. It seems to be an evolved Soul Eater, but not much is clear about this Keyblade. Riku's Keyblade is the Keyblade of the Twilight Realm, or the world in-between. How Riku received this Keyblade is unknown, as he had the Soul Eater in the Land of Dragons, but it had changed to its Keyblade form by the time Riku met Sora again in The World That Never Was.
This keyblade was also seen being held by Ven during the Kingdom Hearts II trailers.
Kairi's Keyblade
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. One of the most reliable reasons would be that Kairi is a Princess of Heart. Tetsuya Nomura has said the scene in which Riku gives her a Keyblade hints at something. In some rumors there has been said it's name is Papairi but this seems very unlogical, probably formed from the words Paopu, and Kairi. Some assume it's the keyblade that represents Destiny Island, thanks to it's floral design.
Artificial
The "dark" Keyblade was a Keyblade made out of the six hearts of the Princesses of Heart. It had the power to unlock peoples hearts and was used by Riku when he was possessed by Xehanort's Heartless. Riku first used it to unleash Maleficient's dragon form, then wielded it in combat against Sora. This Keyblade disappeared when Sora stabbed himself with it to free Kairi's heart. Also, Riku's Dark Keyblade is believed to a fake.
Roxas' Keyblades
Roxas, being Sora's Nobody, also has the ability to use the Keyblade, and utilizes the Twilight Oathkeeper and the Twilight Oblivion Keyblades, Identical Nobody versions of the Oathkeeper and Oblivion keyblades. These have earned him the name "Key of Destiny" from his fellow Organization members. The Oblivion Keyblade was used to represent Sora's memories of Riku and the Oathkeeper Keyblade to represent memories of Kairi.
Terra's Keyblade
The Weapon that Terra uses throughout most of his appearances. He battles Sora with it in Kingdom Hearts 2: Final Mix. Although it seems to be very powerful, he also takes the Kingdom Key in Kingdom Hearts 2'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, 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.
