The Heartless (ハートレス Hātoresu?) are beings of darkness which manifest in two forms, "Pureblood" and "Emblem". While most Heartless are in fact manifest hearts, they behave entirely devoid of emotion, and were thus named "Heartless". The Heartless appear once again in Kingdom Hearts: Child of the Sun as an enemy to the New Organization.
The Heartless are once again active in Kingdom Hearts: Child of the Sun. The New Organization tries to gain hearts from the Heartless without a keyblade, they also study the Heartless to find easier ways to defeat them.
Note- A name in bold denotes a boss.
To be added...