Darkling

The Darkling is a Pureblood Heartless that appears in Kingdom Hearts χ.

Born from Keyblade wielders who succumb to the darkness in their hearts, these Heartless retain the ability to speak and access the Lanes between, but unlike their human forms, they do not draw power from the Book of Prophecies' fragments during battle.

Kingdom Hearts χ
Outside the clock tower in Daybreak Town, and Skuld are attacked by a group of Darklings. The Heartless demand the Keyblade wielders give them their captured Lux, but they flee after a battle when a Nightmare Chirithy appears before them.

The dark Chirithy explains that the Darklings were once keyblade wielders until they let darkness enter their hearts as a result of it taking their Lux away so wielders wouldn't have fight over it.

Later, Nightmare Chirithy absorbs them after a battle.

Kingdom Hearts Union χ[Cross]
After crossing the world line, the Darklings managed to follow them into the data world. When the glitches begin appearing inside the Clock Tower, Player and Ephemer come across Ventus and soon are surrounded by a Darkling who utters "not you..." which confuses the trio. While traversing through the mysterious tunnel, Ephemer and Player encounter it again, with the Heartless muttering the same phrase it did before. They do battle, and the Darkling runs further into the tunnel, with Emphemer and the Player in close pursuit.

In Game Central Station, Ephemer goes after a Darkling entering Hero's Duty. Through following Ralph into the Cy-Bug Sector, Player meets up with Ephemer who was following the Darkling. Defeating the Darkling for the third time and hearing it say "not you", Ephemer is still perplexed on what it could mean. Before they had a chance to confront it, the Darkling gets eaten by a Cy-Bug, fusing them together. It flies off as the pair chase after it.

Finally catching up to it, Player and Ephemer manage to destroy it. Later, in Sugar Rush, after King Candy the Cy-bugs are defeated, Player and Chirithy can still sense that the Darkling is still around and see that it has gotten much stronger since their last encounter. Having enough, Chirithy asks the Darkling why it keeps following them if they are not the ones it wants. The creature doesn't answer and is destroyed once again. However, Chirithy wonders if the Darkling was drawn to the Keyblade in the same way that the Cy-Bugs were drawn to the beacon in Hero's Duty. Chirithy continued wondering that if the Darkling was really after a Keyblade wielder, but not Player or Ephemer, then who?