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 and picking his nose with his tongue.
Leroy is cunning and quite mischievous like Stitch, though isn't quite as immature despite the "age" difference. Where the similarities end is how brutal Leroy is in comparison to Stitch. Leroy isn't above using violence to get things accomplished and quite enjoys tormenting his foes whenever possible. Leroy 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 his 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. He wears a dark blue spacesuit with a triangle consisting of a deep blue border and green center on his chest and each of his sleeves and patches consisting of a deep blue border and dark blue center on each of his legs, black belt with a deep blue square buckle, both black collar, cuffs, and anklets. (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. He is stronger than Stitch and all of the other experiments. He is also 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 `Oe."
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 eventually ordered by Hämsterviel (who paid Jumba a visit) to instead make the latter "a new version of 626." Forced to comply, Jumba created a red, destructive replica of Stitch, and Leroy was born.
Shortly after, Stitch attacked the lab and Leroy was released to battle Stitch. After a long fight ensued between the twin experiments, Stitch had Leroy pinned down, but Pleakley's untimely arrival distracted Stitch long enough for Leroy to bat Stitch into a containment orb.
After Leroy proved to be more powerful than Stitch, the former was cloned by Hämsterviel into an army. The original Leroy then took over the BRB 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 imposter, 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 Leroys. 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 `Oe", which caused all the Leroys to shut down due to the original's fail-safe.
- Leroy is voiced by the same actor who voices Stitch: Chris Sanders.
- Leroy is Experiment 629. In promotional material for the film, Leroy was 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.
- In spite of Leroy & Stitch being Leroy's only media role, he continued to appear in a number of merchandise popular in Japan.
- Leroy is one of the ten known experiments named by someone other than Lilo, the other nine being Yaarp, Gigi, Elastico, Hammerface, Heat, Thresher, Plasmoid, Morpholomew, and Shrink.
- Leroy looks exactly like Stitch, except Leroy is red, his ears have three smooth dents in them, and he's a bit rougher around the edges.
- The scene where a Leroy clone crashes on Earth in Mertle's backyard is the same animation when Stitch first crashed on Earth in Lilo & Stitch.
- When Leroy was defeated, his eyes looked the same exact way as Stitch's eyes were in Lilo & Stitch 2: Stitch Has a Glitch.
- He has all the same powers as Stitch, with a few extra.
- Leroy's color seems to come from Evile (Experiment 627), and may have been based off of Evile's original design and evil attitude as well.
- The climatic battle between Stitch and the experiments and Leroy and his clones could be a reference to the fight scene from Pokemon: The First Movie, with Leroy obviously standing in for Mewtwo. Both are sinister cloned versions created by the series villains (Leroy by Hämsterviel; Mewtwo by Giovanni) from more benevolent creatures (Leroy to Stitch; Mewtwo to Mew) to help them achieve world domination, and the evil clone even creates an entire army of clones to fight their natural counterparts.
- Leroy's creation is also similar to Mephiles the Dark from the video game Sonic the Hedgehog (2006), as both are a copy of an individual made indestructible (Leroy to Stitch; Mephiles to Shadow the Hedgehog), both have the ability to disguise themselves, and both have clones of themselves.