Shadow

The Shadow (シャドウ) is a Pureblood Heartless that is found in all Kingdom Hearts games.

Shadows are the weakest and most common of all Heartless, and are found almost everywhere. Shadows are created when the darkness in an ordinary person's heart is unleashed. Other Pureblood Heartless, like Darkside, can produce or summon Shadows. Despite their weak appearance, they are a threat to normal people, who usually have no way of defending themselves. Even Sora had no means of damaging them until he attained the Keyblade.

Kingdom Hearts
''Heartless that emerge from various places. They sneak up to their enemies and strike them with sharp claws. They are extremely tenacious, often chasing their prey to the end.''

Kingdom Hearts: Chain of Memories
A Heartless that pops up in all kinds of places.

''The Shadow ambles around and attacks without warning. It is completely invulnerable while hiding within the darkness.''

Kingdom Hearts II
It sinks into the ground while moving, so timing is the key to defeating it.

The moment it surfaces, attack away!

Kingdom Hearts

 * Awakening
 * Destiny Islands
 * Traverse Town
 * Wonderland
 * Deep Jungle
 * Olympus Coliseum
 * Agrabah
 * Monstro
 * Halloween Town
 * Neverland
 * Hollow Bastion
 * End of the World

Kingdom Hearts: Chain of Memories

 * Traverse Town
 * Agrabah
 * Olympus Coliseum
 * Wonderland
 * Monstro
 * Halloween Town
 * Neverland
 * Hollow Bastion
 * Twilight Town
 * Destiny Islands
 * Castle Oblivion
 * Bottomless Darkness

Kingdom Hearts II

 * Twilight Town
 * Hollow Bastion/Radiant Garden
 * The Land of Dragons
 * Beast's Castle
 * Underworld
 * Disney Castle
 * Timeless River
 * Port Royal
 * Agrabah
 * Halloween Town
 * Pride Lands
 * The World That Never Was

Kingdom Hearts 358/2 Days

 * Twilight Town
 * Wonderland
 * Olympus Coliseum
 * Agrabah
 * Beast's Castle
 * Halloween Town

Trivia

 * In Kingdom Hearts, Sora turns into a Shadow after using the Keyblade of People's Hearts to release Kairi's heart at the cost of his own. When in this form, Sora can only move and jump.
 * In the regular Chain of Memories, Atlantica is the only world with Heartless, but without Shadows. In Re: Chain of Memories, Shadows are available in Atlantica only if a Bottomless Darkness map card is used.
 * Shadows may possibly be based upon the similarly named enemy from Super Mario RPG, also developed by Square.