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 and when the glitches arrive 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 tunnel Ephemer and Player come across it again with the heartless only saying the same thing and after the battle it proceeds through the tunnel with the pair following close behind.

In Game Central Station, Ephemer goes after a Darkling entering Hero's Duty. Through following Ralph in Hero's Duty, Player meets up with Ephemer who was following the Darkling. Defeating the Darkling for the third time and hearing say "Not you", Ephemer is still perplexed on what it could mean but before they could dwell on that it gets eaten by the Cybug fusing together and flies off as the pair chase after it.

Finally catching up to it, Player and Ephemer manage to destroy it. After King Candy and cybugs are gone, Player and Chirithy can still sense that the Darkling is still around and see that it has gotten stronger from their last encounter. Having enough, Chirithy asks that if they're not the ones it wants, then why does it keep following them but the creature doesn't answer and is destroyed once again. However, Chirithy wonders if the Darkling was drawn to keyblade the same way the cybugs were drawn to the beacon, and yet was curious if the Darkling was after a keyblade wielder but not Player or Ephemer then who.