Although it is not technically a Heartless, the information given when the game is paused says to defeat Doctor Finkelstein's experimental Heartless. The Experiment, along with the Hostile Program, are the only boss characters in the Kingdom Hearts games who are not original Disney characters, Final Fantasy characters, major original characters of the Kingdom Hearts setting, Heartless, Unversed, or Nobodies. Rather, the Experiment is a character within the Nightmare Before Christmas setting who did not appear in the movie but clearly shares the distinctive art style of that film's characters. It is identified in Jiminy's Journal with a red background on its panel, symbolizing its role as an antagonist.
Kingdom Hearts II
To get Lock, Shock, and Barrel to leave him in peace, Dr. Finkelstein began making the Experiment as a playmate for them. He can be seen working on it during the first episode in Halloween Town.
After seeing that Lock, Shock, and Barrel continuously drop the parts they are meant to bring him, and after many attempts to bring the creation to life, Finkelstein believes the Experiment to be a failure and scraps the idea. But, just as he turns around to start new designs, the creation comes to life and attacks the doctor. Leaving Finklestein unconscious in his laboratory, the Experiment wanders out into the woods and into Christmas Town, where it steals Christmas presents from Santa Claus's workshop.
When Sora, Donald, and Goofy arrive in Halloween Town the second time, they are greeted with the sight of Jack collecting an armful of Christmas presents from the ground. Since he is still wearing his "Santa Claus" outfit, they assume that he had stolen the presents to try Christmas once again. Santa believes that too, when Jack comes to return the presents, and he proposes that the only way to prove his own innocence is to track down the remaining presents and catch the real thief.
After retrieving most of the presents from Guillotine Square in Halloween Town, the group thinks up a plan to lure the thief into a trap by planting a large load of presents in Christmas Tree Plaza, with themselves inside the largest box. Jumping out of the box when they hear it coming, they are shocked to see Finkelstein's experiment, whom they believed to be stolen, standing there, attempting to steal the presents. After a difficult battle, the group subdue the Experiment and bring it back to the doctor.
Santa Claus, after hearing about the Experiment, theorizes that the machine was probably looking for a heart. Dr. Finkelstein agrees, stating that he did not give the creature a heart of its own, as he did with Sally. The creature was just hunting one down, believing that the joy of the presents would be enough to give it a heart. Fitting with the theme of the game, the Experiment seems to be an artificial Nobody, not having a heart but constantly hunting one down to become "whole."