The following is a list of known Keyblade Wielders, defined as characters from the Kingdom Hearts series that have wielded a Keyblade in battle successfully. Characters who have held a Keyblade only for it to return to the hand of its true holder are not included in this list, as a Keyblade will typically refuse to be wielded by anyone without a strong enough heart.
Sora
The Kingdom Key is Sora's default Keyblade, which appeared to him as his home world, Destiny Islands, was being consumed by the darkness. A Keyblade from the Realm of Light, its default keychain is a small set of three silver circles that form the symbol most commonly associated with Mickey Mouse. It is commonly referred to within the game as, quite simply, "the Keyblade". Sora is also able to summon his Keyblade 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.
Sora's key is repeatedly shown to possess the ability to seal Keyholes, though it is yet to be revealed as to whether any other Keyblades possess this same power, however it is presumable that they do, as the King's Keyblade was used in tandem with Sora's to seal the Door to Darkness at the end of Kingdom Hearts.
Riku
Riku and Sora were both entitled to the Kingdom Key of the Realm of Light, but when Riku chose darkness over light, the darkness weakened his heart, causing the Keyblade to pass on to Sora. In Hollow Bastion, Riku successfully acquires Sora's Keyblade, but ultimately loses it again when Donald and Goofy, who were under orders from King Mickey to follow the Keyblade Master (which was now Riku), chose their friendship with Sora over the King's orders, which strengthens Sora's heart and allows him to reclaim the Keyblade. Riku came to wield another Keyblade during the course of Kingdom Hearts, while possessed by Xehanort's Heartless.
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. It is speculated that the Keyblade could have been used as a median considering its 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.
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 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 Tetsuya 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. Nomura has said the scene in which Riku gives her a Keyblade hints at something. It is assumed it's the Keyblade that represents Destiny Islands, given its floral design and the Paopu Fruit Keychain.
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 Keyblades that Roxas used and the memories 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.
Roxas can use Oblivion and Oathkeeper to perform a "Clear Light" attack. Nomura stated that Roxas' ability to use two Keyblades means that Sora can, too, without the Drive Forms, due to the fact that Roxas has every ability that Sora has. Nomura has also 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.
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. The replica model letter opener called Mickey's Keyblade "Darkside", which is fitting as it is the Keyblade of the Realm of Darkness. Mickey used this Keyblade along with Sora's to seal the Door to Darkness, so it theoretically has the same powers as its Realm of Light counterpart.
Prior to that, Mickey originally possessed the Star Seeker, which is seen in 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 also appears to wield the Star Seeker in Kingdom Hearts coded.
Xehanort's Heartless
Made from the hearts of six of the Princesses of Heart, Xehanort's Heartless' Keyblade has the power to unlock people's hearts and send them to darkness, reflected in the heart-emblem on its point. He wielded this Keyblade while possessing Riku, but is unable or unwilling to conjure or use the Keyblade himself. This Keyblade was destroyed when the hearts of the princesses were restored. The Keyblade is referred to as the "Dark Keyblade". It is also called "Artificial" by some fans because it was created by people's hearts so it is technically not a true Keyblade. 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. Afterwards, Sora impales himself through the chest with the Dark Keyblade, restoring Kairi's heart at the (temporary) cost of his own. Like Terra, Aqua, and Ven's Keyblades it lacks a chain. This may be due to the fact that it was an artificially made Keyblade.
Terra
Terra battles Sora with this Keyblade 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 possess, 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 formations.
Terra is shown 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. The Keyblade's name is not yet known.
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 is shown 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 Final Mix+. Whether or not this Keyblade has a name is as of yet unknown.
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 Terra and Aqua's, it lacks a keychain.
When Master Xehanort freezes Ven and drops him off the cliff in the Final Mix+ secret video, the Keyblade (which was frozen as well) shatters in half when it smashes into the rocks. Some theories speculate that when it shatters, one half (the handle) makes the Kingdom Key but this has been denied because in the video of Sunset Horizons Terra grab the kingdom key meanwhile Ven grab his own keyblade and the Way to Dawn. It is unknown if this Keyblade has a name or not.
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 black blade, a black hand-guard and a head resembling a battle axe. It resembles the Oblivion keyblade somewhat, but with two shafts and a different head. What power his Keyblade holds is shrouded in mystery; however, upon dropping Ven off of the cliff, he converts his Keyblade into an orb of dark energy, which is shot into the sky to summon Kingdom Hearts. His Keyblade possesses 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 and his Master's Keyblades and holds great strength in Thunder and Blizzard attacks. His Keyblade possesses a keychain. His Keyblade, is red with a round, cog-like shape on the end with green eyes on it.
Genie
Genie from Aladdin also wields a Keyblade (or facsimile) in Kingdom Hearts II, as his attacks mirror Sora and his forms. His "Keyblade" partly resembles Three Wishes from the first game. He seems to obtain it by tapping on Sora's Keyblade when summoned.
Xion
Xion, the fourteenth member of Organization XIII, has recently been to confirmed to wield a Keyblade. Riku pondered over the possibility of her, being a Nobody, using a Keyblade, even going as far as to call the Keyblade a fake. It has recently been revealed that the Keyblade that she wields is the Kingdom Key. Since this is a Keyblade that has been seen before, some speculate that Riku was reluctant to believe someone else could have a Keyblade.
Plot Hole
Theoretically, since Sora could not damage Heartless without the Keyblade, and it is said that the Keyblade is the weapon that destroys Heartless, that should mean that no one except Keyblade Wielders should be able to destroy Heartless, however fellow party members and other characters can kill them as well. A possible explanation for this is that without the Keyblade, Sora was not strong enough to even damage the Heartless. Another possibility is that the power of the Keyblade is channeled through any hearts aligned with Sora, allowing his party members to wield a small part of its strength. This theory seems to be lent a little more weight by the fact that before meeting Sora, Donald and Goofy were unable to defeat a pair of small Heartless but as soon as they allied with Sora they were instrumental in the defeat of a rather large Heartless.
Another mistake is the fact that the Keyblade is supposed to be the only weapon that can release hearts when defeating the Heartless, though it is shown that other people can release the hearts. Besides party members being able to release hearts in battle, Sephiroth and Leon are shown to do that in cutscenes. However, "destroy the Heartless" could have a less literal definition, as it might mean that the Keyblade is the only weapon capable of bringing permanent extinction to the Heartless, given its unique ability to seal dimensions and worlds.