Like Stitch, Experiment X21 is very destructive and mischievous. He is also a physical, obedient, intelligent, cunning and extremely aggressive creature.
Experiment X21 is a purple koala-like experiment with a strong resemblance to Stitch, indigo nose, claws and toes, larger ears, lavender fur on his belly, chest, and around his eyes, and dark indigo markings on his back, and tips of his antennae, ears and back spikes.
Like Stitch, X21 is able to mutate into second form with four arms, two long antennae on top of his head, and three sharper spikes on his back. He also possesses all of Stitch's abilities of brain, strength, speed, invulnerability, and fighting skills, to lift objects up to 3,000 times his weight and not an ounce more, think faster than a supercomputer, have super sight and hearing, be bulletproof and fireproof, have several different visions, including night vision and heat vision, cling to walls and walk on the ceilings, like a gecko, roll into a ball, hear in different hearing levels, have venomous spikes, and eat stone, metal and flesh. He also possesses other abilities such as flight, teleporting, invisibility, telekinesis, laser vision, plasma cannon blast, plasma shot, plasma bolts and webs, ice breath, fire breath, sonic roar, electric shock, energy blast and electricity.