The Experiment

The Experiment is an artificial being created by Dr. Finkelstein. It is a boss of Kingdom Hearts II, and is fought in Halloween Town. Although not a Heartless, it resembles the Guard Armor Heartless of the first Kingdom Hearts, in the fact that it is comprised of many different parts that can disassemble and fight separately, or reassemble into a more powerful entity.

Kingdom Hearts II
Dr. Finkelstein's puppet creation.

Unlike Sally, it didn't have a heart -- so it went to find one in the love that's bundled with all presents.

Story
The Experiment plays a large role in the story of the second visit to Halloween Town. It is a large, humanoid machine that Dr. Finklestein is making as a playmate for Lock, Shock and Barrel so that they would leave him in peace. During the first visit, the doctor is seen working on it, and it lies on the operating table in his lab throughout the first visit.

After seeing that Lock, Shock, and Barrel continuously drop the parts they are meant to bring him, and after many attempt to bring the creation to life, Finklestein 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 wandered out into the woods and into Christmas Town, where it stole Christmas presents from Santa Claus' 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. As he is still wearing his "Santa Claus" outfit, they assume that he had stolen the presents to try Christmas once again. Santa believed that too, when Jack came to return the presents, and he proposed that the only way to prove his own innocence was 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 Finklestein'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. Finklestein 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."

In Battle
The Experiment acts similarly to Guard Armor and Opposite Armor in battle. While at first being one large enemy with a variety of attacks, it eventually separates into smaller enemeies composed of different combinations of its parts. Little damage can be done to the parts individually, but the use of carefully timed Reaction Commands can bring it back together, and it can be defeated in this way.

It uses combinations of laser and shockwave attacks against Sora, and uses wind-based attacks to knock back Sora, keep him airborne, or prevent him from attacking. The Kickspring Reaction command counters the Experiments attacks and is the only way to bring it back together to defeat it.

Trivia

 * The Experiment is one of only three bosses that is not a Heartless, Nobody, Disney Character, or Final Fantasy Character, the others being Riku (Enemy) and the Hostile Program.
 * Although it is not a Heartless, the information given when the game is paused says to defeat Doctor Finkelstein's experimental Heartless.