Kingdom Key

The Kingdom Key is the most iconic Keyblade in the  series, as it appears within each game of the series, and frequently serves as the default Keychain for cutscenes. In Kingdom Hearts Chain of Memories, it is the only Keychain that Sora can visibly use, while in Kingdom Hearts 358/2 Days, it is the only Keychain of Xion's to appear in Story Mode, and excluding the final Mission, the only Keychain of Roxas's to appear in any cutscene. Furthermore, while Kingdom Hearts II normally depicts the equipped Keyblade within cutscenes, the Kingdom Key appears by default in the cutscene leading up to the Battle of the 1000 Heartless, and within the Album pictures of Jiminy's Journal. In Kingdom Hearts 358/2 Days, it can be upgraded to the Kingdom Key+ and the Kingdom Key++.

The Kingdom Key draws out the true power of Sora's Keyblade, even though it is not the strongest Keychain. As such, it is the true form of his Keyblade, which means that it is also the default form of Roxas's and Xion's Keyblades. The Kingdom Key is not wielded by Sora exclusively; Riku, Roxas, and Xion have also used it, and King Mickey is often seen wielding the Kingdom Key D, a color-inverted version of the weapon.

For Roxas and Xion, the Keyblade they wield is in fact Sora's Keyblade, existing in multiple places at once. Roxas also obtains a second Kingdom Key after absorbing Xion; however, this is in fact Ventus's Keyblade, not Xion's.

The Kingdom Key D, a color-inverted Kingdom Key which King Mickey wields, is the Keyblade from the Realm of Darkness, which he obtained to mirror Sora's Keyblade from the Realm of Light.

Kingdom Hearts
When Riku opens the door in the Secret Place and allows the Heartless to invade, he chooses to accept the darkness and vanish into a portal of darkness. However, Sora touches the light in Riku's heart, forging and gaining access to the Kingdom Key in the process. Riku, as the Kingdom Key's intended wielder, is able to hold it briefly when he meets up with Sora in Traverse Town, but returns it to Sora.

When Sora arrives in Hollow Bastion to rescue Kairi, Riku reveals to Sora that Sora's Keyblade is meant to be his own and summons it to his hand, where it reverts back to the Kingdom Key. However, when Sora later confronts Riku again, the strength of his heart allows him to make the Keyblade his own.

Kingdom Hearts Chain of Memories
Due to the castle's rules, Sora cannot switch Keychains while in Castle Oblivion and is stuck with using the Kingdom Key, although he is able to channel the abilities of his past Keychains by using Attack Cards.

When Zexion disguises himself as Sora, he wields a fake Kingdom Key.

Kingdom Hearts 358/2 Days
Roxas and Xion wield the Kingdom Key as members of Organization XIII, and when he loans his Keyblade to her, she uses this form exclusively.

When Roxas first uses Synch Blade, he summons two Kingdom Keys, then transforms them into the Oathkeeper and Oblivion.

Kingdom Hearts II
When Roxas is attacked by a lone Dusk outside the Old Mansion, he attempts to use his Struggle Bat to ward it off, but it fails to touch the monster. At that moment, however, the Kingdom Key replaces the Struggle Bat in a haze of data, and Roxas is able to defeat the Dusk. The Kingdom Key continues to materialize whenever Roxas needs to fight off Nobodies, until he is attacked by Axel within the Old Mansion's Basement Corridor. There, Roxas instead summons the Oathkeeper and Oblivion, but after defeating Axel and proceeding to the chamber in which Sora slumbers, he uses the Kingdom Key one final time in an attempt to strike down DiZ.

Sora continues to wield his Kingdom Key throughout his adventure.

Kingdom Hearts coded
Data-Sora gains a data-version of the Kingdom Key in his Dive to the Heart. It is destroyed by Maleficent at Agrabah, but he gains a new Keyblade at Hollow Bastion. According to King Mickey, this Keyblade is closely connected to the power of the Heart.

Kingdom Hearts 3D: Dream Drop Distance
Sora wields the Kingdom Key throughout his Mark of Mastery, first using it to subdue Ursula. When Sora is corrupted by the Armored Ventus Nightmare, it wields a blackened Kingdom Key in a reverse grip, similar to Ventus. When Riku defeats the Nightmare, the Kingdom Key reverts to its original colors and Riku uses it to lock Sora's heart, then to open a portal back to the outside world.

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage
Sora uses the Kingdom Key to open a passageway to Olympus Coliseum.

Kingdom Hearts III
Sora continues to use the Kingdom Key as his main weapon. The Kingdom Key is also used by Xion.

Design
The Kingdom Key is depicted as a classic approximately 3.5' long, with a long silver Keychain extending from the hilt, and a  token on the end of the Keychain. In Kingdom Hearts, the token is formed from three spheres, while in other games the token is flat and seemingly cut from sheet metal. The blade and teeth of the Keyblade are silver, the rainguard is blue, and the guard is bright gold. The teeth of the Keyblade form the outline of a crown which matches the crown charm Sora wears.

Gameplay
Gameplay with the Kingdom Key varies, as its combo is normally influenced by the amount of Combo Boosts and other Combo influencing abilities the player has equipped. The weapon's base combo in nearly every game is a basic three hit attack: A downward swing, a forward thrust, and a turning sweep slash, while its aerial combo is a downward, then upward slash, finished with a mid-air spinning slash.

In Kingdom Hearts 358/2 Days, Roxas, Xion, and Sora have the opportunity to wield this weapon: Roxas and Xion, with no gears or the Zero gear equipped, and Sora, with any gear save for the Zero and Pandora gears.

Roxas and Xion have identical combos with both no gears and Zero gear. Their combo without any gears is the basic three hit combo, while his Zero gear combo is slightly different. It begins with a downward slash, followed by an upward swing with the capability of knocking the enemy in the air. From here, a -Combo is prompted, in which Roxas or Xion will perform a jump accompanied by a downward slash if executed. With another hit of the button, Roxas or Xion's -Combo concludes with a sliding swing of the Keyblade. If continued normally, combo concludes with a basic turning slash. The aerial combo for Roxas's Zero Gear is noticeably shorter, with only two hits. It begins with a basic downward slash and finishes with a spinning slash.

Sora's combos with the Kingdom Key, meanwhile, never change no matter what gear is equipped. It will always be the same, default, three-hit combo: a sideways swing, a forward thrust, and a turning slash. No -Combo opportunities are ever prompted. His swing speed, however, is noticeably faster than Roxas's.

Other appearances
Most Kingdom Hearts merchandise features the Kingdom Key as Sora's weapon, including the Volume One and Volume Three Sora figures that appear in the Formation Arts Figures. Along with being the Keyblade packaged with "normal" Sora in the Series 1 Play Arts Figures, it is also seen with Roxas's figure and the Sora Limit Form figure.

Another side, Another story...
At Memory's Skyscraper, Roxas briefly dual wields the Kingdom Key and Oathkeeper after tossing his Oblivion to a falling Riku.

The Gathering
At the Keyblade Graveyard, Terra pulls the Kingdom Key from the ground alongside Aqua and Ventus.

Kingdom Hearts Birth by Sleep
In the opening video for Kingdom Hearts Birth by Sleep, the Kingdom Key is briefly shown after Terra and Aqua, surrounded by many other Keyblades, are seen falling towards Ventus's Dive to the Heart station. The Kingdom Key and the Soul Eater land blade first into the station, shattering it into glass like pieces and revealing Ventus amongst them.

The logo sequence for the Keyblade Graveyard also depicts the Kingdom Key being thrust into the background, summoning the text.

Kingdom Hearts 3D: Dream Drop Distance website
On the Japanese version of the Kingdom Hearts 3D: Dream Drop Distance website, the Kingdom Key acts as a cursor for Sora's version of the website.

Disney Infinity 3.0
The Kingdom Key is the game's "ultimate unlock," and can be used if the player owns every available figurine for the game.

World of Final Fantasy
The Kingdom Key is wielded by Sora.''

The Simpsons Game
The Kingdom Key can be seen in the "Big Super Happy Fun Fun Game" level, where a statue of holds it in his mouth.

Final Fantasy Record Keeper
The Kingdom Key appears as a 5★ Keyblade. It can be acquired from the Key to Another World Event Dungeon as well as a Login Reward during the Kingdom Hearts collaboration event.

Face My Fear
The EP cover art for "Face My Fears" depicts Utada Hikaru wielding the Kingdom Key in her left hand.