The dark creatures that Sora fights in Kingdom Hearts are called the Heartless.
Heartless are beings created when darkness consumes a person's heart. They are the most common enemies in the series and are driven by an instinctive desire to find and consume other hearts. Their name refers to their lack of a conventional heart, even though they originate from the darkness within hearts.
The first game distinguishes between naturally occurring Pureblood Heartless and artificially created Emblem Heartless. The familiar Shadow enemy is a Pureblood, while many Emblem Heartless carry a recognizable heart-shaped insignia. Sora uses the Keyblade to defeat them and release the hearts they have stolen.
They are often confused with Nobodies, Unversed, or Dream Eaters, but those are separate kinds of beings introduced in different Kingdom Hearts stories. Nobodies are associated with the body and soul left behind when a strong-hearted person becomes a Heartless, while Dream Eaters primarily appear in the sleeping worlds of Kingdom Hearts 3D: Dream Drop Distance.