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. One legend states that the Keyblade master saved the world, while another said he wrought chaos and destruction. While this legend may appear to contradict itself, it can be assumed that the Keyblade master chooses how to use the Keyblade, whether for good or evil.

Appearance
All Keyblades shown have the same distinguishing features that are modeled after a key: a guard with a handle, a hilt and a blade with some type of a sharp point that gives it a cutting ability, which comes from the grooves and teeth of a real key. All Keyblades also have some theme that the keyblade is modeled after, which usually relates to the keychain attached.

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.). A known exception to the rule is Roxas, a Nobody (and therefore without a heart). However, he is able to wield Keyblades because he is Sora's Nobody.

Theories
There are many theories surrounding the Keyblade that include what a Keyblade's purpose is, how a Keyblade is formed, and the significance of a keychain. Various complications occur when the crossroad with Keyblades with no keychain from previous games were shown. While the origins of the keychains are a mystery, there are many theories to its purpose, such as that they are charms that summon the keyblade to its 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 his own darkness. 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 could be said for Keyblades. Yet another theory is that the Keychain is a physical embodiment of one's memories, which would explain why Sora got new Keyblades after visiting every world. It also explains Roxas's Keyblades; because Roxas is Sora's Nobody, Roxas's Oblivion represents Sora's memory of Riku, and his Oathkeeper represents Sora's memory of Kairi. According to some fans, the Keyblade is also the mysterious voice which guides Sora and Roxas in the beginning and ending of both games.

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, Riku's, and Kairi's Keyblades do have keychains, so they MAY be able to do so in theory.

Sora
The Kingdom Key is Sora's Keyblade. Sora's Keyblade was given to him in the dying minutes of his home world, Destiny Islands, when it was consumed 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 strongest heart use it, usually Sora, and instantly returns to that person's hand when taken by another. Sora is also able to summon it back in a flash of light, as seen during his opening conversation with Leon in Traverse Town and during his fight against Roxas in The World That Never Was.

Riku
Riku was the true wielder of the Kingdom Key of the Realm of Light, but when he chose darkness over light, the darkness weakened his heart, causing the Keyblade to pass on to Sora. Riku came to wield another Keyblade during the course of Kingdom Hearts, while possessed by Xehanort's Heartless, which is described below.
 * Riku's Keyblade:

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. The Way to the Dawn is modeled to reflect Riku's abilities of both light and darkness, bearing angelic and demonic wings on both its blade and its handle. It also appears to have multiple crosses lodged in the Keychain, and it bears a heart symbol that closely resembles the Heartless emblem. Finally, it bears a resemblance to Oblivion, the Keyblade that is supposed to represent Riku.

This Keyblade appears in the secret endings of Kingdom Hearts II and Kingdom Hearts II: Final Mix+, meaning that it was created before the Soul Eater. This suggests the Soul Eater is based on this Keyblade; or instead unrelated.

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 floral 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 its floral design and the paopu fruit Keychain.
 * Kairi's Keyblade:

King Mickey
King Mickey’s Keyblade is a Keyblade from the Realm of Darkness, which the King received 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 theoretically 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:

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. Furthermore, these two both used the keyblade at the same time. In order to learn why he could wield the Keyblade, Roxas 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 represent Riku and Kairi, Riku being Oblivion, and Kairi being Oathkeeper. All of these things combined (when Roxas fuses with Sora) complete 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). 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, among other things.

In a recent trailer leaked on to the internet, we see Terra summoning his Keyblade differently than how most wielders do; his appears surrounded by translucent blocks that are also seen in some of his attacks from final mix+, in the form of a flash of light.

Aqua
Fittingly, Aqua wields a blue Keyblade (which has a slight resemblance to Oblivion). Like Terra's, it lacks a keychain. She prefers to use magic with it, as evidenced in the secret ending, where she skillfully conjures Blizzard, Thunder, and Reflect. Her Keyblade is also seen along with her empty, damaged armor in a new scene in Kingdom Hearts II: Final Mix.

Ven
Ven's Keyblade is uniquely designed to be held in reverse (although, in the gameplay trailer, he is seen holding it the normal way). Like the other two, it lacks a keychain. The Keyblade has slight resemblances to both the Kingdom Key and the Jungle King. When Master Xehanort freezes Ven and drops him off the cliff in the secret ending, the Keyblade (which was frozen as well) shatters in half when it smashes into the rocks.

Master Xehanort
It is confirmed that Master Xehanort wields a Keyblade as well as his apprentice. His Keyblade has a double-edged appearance with a separated white/blue blade and a dark red gear-like emblem as a handguard (making it vaguely similar to the Ultima Weapon), and he wields it efficiently in both hands. What power his Keyblade holds is shrouded in mystery; however, upon dropping Ven off of the cliff in the Final Mix + Teaser Trailer, he converts his Keyblade into an orb of dark energy, which is shot into the sky to summon Kingdom Hearts. His Keyblade possesses to a keychain.

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 holds great strength in thunder and ice attacks. He might have a connection to Riku. His Keyblade possesses a keychain.In the Secret ending of Kingdom Hearts 2 final mix+, it shows the Keyblade, which is red with a round, cog-like shape on the end.

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 "3 wishes". He obtains this by tapping on Sora's Keyblade during the Summon sequence in Kingdom Hearts II.

Ansem (Xehanort's Heartless)
Made from the hearts of six of the Princesses of Heart. It has the power to unlock people's hearts and send them to darkness, reflected in the heart-emblem on its point. Xehanort's Heartless wielded this Keyblade while possessing Riku, but is unable or unwilling to conjure or use a Keyblade himself. This Keyblade was destroyed when the hearts of the princesses were restored. The Keyblade is referred to as the "Dark Keyblade" (also called "Artificial", as it has no official name, and is rarely referred to as the "Heartbreaker" to give it a more formidable name). Riku first used it to unleash Maleficent's dragon form, then wielded it in combat against Sora, where he then lost it after the battle.