Shadow

Shadows are an enemy from all Kingdom Hearts games, and have also been seen in screenshots of the upcoming game, Kingdom Hearts 358/2 Days.

Shadows are the weakest and most common of all Heartless. They are Pure Heartless found almost anywhere, though their hot spots include the Destiny Islands, Traverse Town, Wonderland, End of the World, and The World That Never Was. Shadows are created when the darkness in an ordinary person's heart is unleashed. Other pure Heartless, like Darkside, can produce Shadows or at the very least, summon existing ones.

Despite their weak appearance, they are a threat to regular people as they usually have no way of defending themselves. Even Sora had no means of defeating them, until he attained the Keyblade, since without it he can't inflict any noticeable damage to them.

There is a higher "class" of Shadows called Neoshadows, who are far more organized and dangerous. Shadows also have Nobody counterparts called Creepers.

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!"

Trivia
In Kingom Hearts 1, Sora turns into a shadow after waking Kairi after she lost her heart. When in this form, Sora can only move and jump.