Leroy is an antagonistic, intelligent, conceited, rude, small-minded and shapeshifting experiment who is Hämsterviel's henchman and Stitch's archenemy and evil twin. Despite his loyalty to Hämsterviel, Leroy enjoys seeing him suffer, as he is shown laughing at Hämsterviel when the latter falls to the ground after their cannon is destroyed.
Like Stitch, Leroy is shown to be very destructive, but in a more vicious than mischievous way. He is also shown to be a physical, sadistic, cruel, and disgusting slob. Despite this, he also possesses some of Stitch's habits, including holding soda cans in his mouth, picking his nose with his tongue, and sharing Stitch's love for Elvis Presley's music, as Leroy and his clones were dancing to "Jailhouse Rock" in their prison cells.
Leroy is cunning and quite mischievous like Stitch, though he isn't quite as immature despite the "age" difference. Where the similarities end in Leroy is how brutal he is in comparison to Stitch. Leroy isn't above using violence to get things accomplished and enjoys tormenting his foes whenever possible. He will more often than not rely on brute force in battle and tends to not show mercy to his opponents. While he does rely on brute force, Leroy is capable of planning out his attacks and is quite smart. While his English isn't perfect, Leroy can speak it a bit better than most other experiments.
Leroy resembles an exact duplicate of Stitch with rougher, red fur, a mahogany nose, black claws, toes, tips of antennae and ears and back markings, a light tan/pinkish-orange stomach from his chin down and around his eyes, crimson markings on the back of his head and on his back, frilly ears with light purple inside, yellow teeth, a fluffy tail, and a slightly deeper voice as well as two extra, retractable arms, two bent retractable antennae on his head, three bent retractable spines tipped with white and a very dark mahogany on his back, and retractable claws on his front and back paws. When his mouth is closed, his canine fangs stick out. (Note: Leroy can be considered as Experiment 629, as he was the 629th experiment created.)
Leroy possesses all of the same powers as Stitch, with a little extra boost of strength and speed, along with the ability to change his fur color to pose as Stitch and instantly regrow his fur. He can curl into a ball and roll with incredible speed, dealing heavy damage to anything he hits. He is fireproof, bulletproof, shockproof, acid-proof, explosion-proof and plasma-proof. He is also smart, faster than a supercomputer, and can absorb electric and fire powers. He is immune to Angel's forward song and stronger than Stitch and all of the other experiments. He is normally very physical, but also good with a plasma gun.
During his creation, Leroy was programmed with a fail-safe that causes him and his clones to shut down when they hear the song "Aloha ʻOe".
It is unknown whether or not Leroy shares Stitch's weakness of water.
Experiment 629 was the 629th and last genetic experiment created by Jumba with Hämsterviel's funding. He was designed to have all of Stitch's powers, with a little extra boost of strength and speed. He was named Leroy by Dr. Hämsterviel.
While spending a few years on Earth, Jumba had an unfinished experiment at his laboratory in deep space. After Lilo and Stitch had successfully captured all 625 experiments and turned them from bad to good, Jumba regained access to his lab. During this time, Jumba's evil former partner Dr. Hämsterviel escaped prison with Gantu's help.
Jumba started creating an experiment with explosive saliva from a template similar to Stitch until he was ordered by Hämsterviel, who paid Jumba a visit, to instead make the latter "a new version of 626." Forced to comply after being threatened by Gantu, Jumba created a red, destructive replica of Stitch, and Leroy was born.
Shortly after, Stitch attacked the lab to apprehend Hämsterviel, and Leroy was released to battle Stitch. After a long fight ensued between the twin experiments, Stitch eventually had Leroy pinned down, but Pleakley's untimely arrival distracted Stitch long enough for Leroy to bat him into a containment orb.
After Leroy proved to be more powerful than Stitch (although he technically didn't win fairly), the former was cloned by Hämsterviel into a huge army. The original Leroy then took over the B.R.B. by posing as Stitch, the captain, and ordered that the pilots take him, Hämsterviel, and the Leroy army to Turo.
En route, Leroy received a call from Lilo and, resuming his Stitch disguise, attempted to pose as Stitch. However, he was quickly discovered by Lilo to be an impostor, as he was not wearing the tiki necklace that she had given Stitch, causing Hämsterviel to order Leroy to "hang up" (namely by blasting the communicator).
Upon arriving on planet Turo, the Leroy army was able to overthrow the council and gain galactic domination.
Shortly after, Hämsterviel and the entire Leroy army arrived at the stadium on Earth in the BRB to destroy the captured experiments. Before they could, however, Lilo, Stitch, Jumba, Pleakley, Reuben and a reformed Gantu arrived, and a battle ensued between the experiments and the Leroy clones.
Though the experiments were able to overpower the Leroys at first, the Leroys soon gained the upper hand. However, as the Leroys came close to winning the battle, Lilo, Stitch, Reuben and several other experiments performed the song "Aloha ʻOe", which caused all the Leroys (including the original one) to shut down due to the original's fail-safe.
All of the Leroys were then sent to prison on Asteroid K-37, where they were seen happily dancing to the song "Jailhouse Rock" with an angry Dr. Hämsterviel tapping his foot.
- Leroy is Experiment 629, although in promotional material for the film and on some Japanese and Disneyland Paris merchandise, Leroy was/is also referred to as Experiment 628. However, this is false as he was only called Leroy in the film and is the evil doppelganger of Experiment 626.
- It is never said or shown what Experiment 628 is.
- Leroy is one of the ten known experiments named by someone other than Lilo, the other nine being Gigi, Elastico, Yaarp, Hammerface, Heat, Thresher, Plasmoid, Morpholomew, and Shrink.
- When Dr. Hämsterviel christened the newly-created experiment Leroy, everyone laughed at the name except Lilo, who thought Leroy was a fine name.
- Leroy looks exactly like Stitch, except Leroy is red, has smooth dents in his ears, and is a bit rougher around the edges. He also has all the same powers as Stitch, with a few extra.
- Hämsterviel wanted Leroy's fur color to be red instead of blue so as to match his cape, and that he doesn't like the color blue.
- Ironically, Leroy can change his fur color to look like Stitch's.
- According to the DVD scene selection, Leroy has about 50,000 clones.
- The scene where a Leroy clone crash lands on Earth in Mertle's backyard is the same animation when Stitch first crash landed on Earth in Lilo & Stitch.
- When Leroy was defeated, his eyes glowed green as Stitch's eyes did in Lilo & Stitch 2: Stitch Has a Glitch.
- Leroy's color seems to come from Experiment 627, and may have been based off of 627's original design and evil attitude as well.