Keychain

A Keychain (キーホルダー) is a magical piece of equipment that modifies the appearance and abilities of a Keyblade. In Kingdom Hearts 358/2 Days, the Gear Panel function replaces Keychains, and in Kingdom Hearts Re:coded the Keyblade Chip in Gear Matrix replaces Keychains. Likewise, the Card system replaces Keychains in Kingdom Hearts: Chain of Memories. Cards, however, do not affect appearance, only ability.

Power
The Keychains have the ability to change a Keyblade's stats, grant it special abilities, and give it a new appearance. While most Keyblades have a basic form, like the Kingdom Key, a Keyblade wielder can upgrade and change the form of the Keyblade with a Keychain. In other words, when Sora or another wielder is equipping a "new" Keyblade, he or she is equipping a different Keychain to the Keyblade, changing its form.

Origin
It has not yet been explained exactly what a Keychain is and why it has the power to transform the Keyblade. A wielder usually receives a Keychain after "completing" the events that transpire within a world, but the Keyblade wielder is not shown receiving the Keychain from anyone, save for when Tifa hands Sora the Fenrir Keychain after defeating Sephiroth, when Kairi hands Sora her Wayfinder, which turns out to be the Oathkeeper Keychain, and when Ariel gives Sora the Crabclaw Keychain, as well as the few Keychains found in chests. These Keychains are often modeled after elements of the world from which they are received in some way, and nearly all of his Keychain tokens represent some important item from a world he has saved. Usually, Sora is not actually shown to be given the Keychain from one of his many friends, so it is possible that after saving a world, or completing a major step in that direction, the world itself rewards him with the ability to access its power in the form of a unique Keychain.

Keychains that Sora receives from his friends, or from defeating challengers in the Olympus Coliseum, always use a token that is somehow intimately symbolic of that person. While Sora's Keyblade is not specifically a Keyblade of human hearts, like the explicitly named Keyblade of People's Hearts, it is likely that the connections between his heart and those he meets allow him to sometimes access their power as well. Sora and Roxas's Oathkeeper and Oblivion are said to specifically embody Sora's memories of Kairi and Riku and Roxas's oath to Xion before her death (Oathkeeper) as well as her demise (Oblivion) and so it is extremely likely that for "people-Keyblades" like these, the Keychains act as a physical embodiment of the memories Sora and Roxas have of those that are important to them. Keyblades that represent entire worlds likely function the same manner, being formed from Sora's memories of that world.

Roxas utilizes Gears, which function more or less exactly the same as Keychains would, and create Keyblades that feature apparent Keychains.

Sometimes, a Keyblade wielder may also obtain more powerful forms of their personal Keychain, usually after having some kind of epiphany; for example, Aqua's Rainfell became Stormfall, Terra's Earthshaker became Ends of the Earth, Riku's Soul Eater became Way to the Dawn, and Roxas's two Kingdom Keys became the Oathkeeper & Oblivion. Despite these being treated as an upgrade to a specific Keychain, the wielder can still use the original Keychains at will.

Each Keychain has its own innate strengths and abilities.

Most Keyblades also have some theme, usually a world or a person associated with that world, that will reflect in its appearance. A Keyblade's appearance is primarily determined by the Keychain attached to it.

Each Keyblade has a unique effect when it is summoned, which usually matches the Keychain's symbolism. For example, when Oblivion is summoned, a dark aura and violet sparks surrounds Sora's hands before the Keyblade actually appears. The particle effects that appear when the Keyblade is used to hit something also change with each Keychain. For example, the Keyblade "Two Becomes One" uses black and white checkered crowns when a solid object is hit.-->

Exceptions
Sometimes, a Keyblade does not possess a Keychain. For example, the incomplete Keyblade of heart was missing a Keychain, as are the abandoned Keyblades which litter the Keyblade Graveyard.

The Ends of the Earth, Wayward Wind and Stormfall did not posses Keychains in the Kingdom Hearts II secret video Birth by sleep, as they were not fully designed yet.