Experiment 621 was the 621st genetic experiment created by Jumba with Hämsterviel's funding. He was presumably designed either as a prototype of Stitch and Reuben, or for Stitch/Reuben's purpose. His original purpose was to collect strands of DNA around the galaxy to help Jumba create more experiments.
621 had been sent around the galaxy to collect DNA strands to help Jumba create more experiments; however, Jumba had been disappointed with his results.
Jumba later gave this mission to 626 and was more satisfied with 626's achievements. Jealous of Experiment 626 because of all the attention Jumba was giving him, 621 secretly collected his own DNA and used it to mutate himself into a bigger, stronger form. 621 then battled 626, but was defeated when 626 tricked him into electrocuting himself. The unconscious Experiment 621 was apprehended by Gantu along with Jumba and 626 shortly after.
Experiment 621 is shown to be sadistic, prone to jealous hatred and possesses a superiority complex, shown when he became obsessed with proving he was "superior" to 626 which included trying to kill him. However, this attitude can be somewhat justified, since Jumba harshly criticized and completely neglected 621 when 626 proved to be a more effective experiment.
621 may be mentally unstable due to his unusual tendency to burst into random laughter and because of his merciless rage. Despite this, he appears to be more intelligent than most experiments, including his ability to speak fluent English.
621 does not appear to be a very capable fighter, only fighting his battles with sheer strength rather than skill. He can also be quite reckless and clumsy in such circumstances.
In his regular form (not counting his extra limbs), 621 resembles a slim, dark green koala standing on two, very slim legs. The fur on his belly, bottom jaws, and around his eyes, however, are a dull yellow color. On his rather large-in-proportion head are a pair of bugging black eyes, a large black-green round nose, and a wide underbite with two jutting fangs. His ears are quite long and slim, almost like a bat's own ears, and has creamy yellow insides. He is quite short, standing at three feet, including his dark yellow spiky Mohawk.
He wears a dark brown sleeveless space suit with a circle consisting of a darker brown border and yellow center on his chest and patches consisting of a brown border and yellow center on each of his legs, black belt with a yellow square buckle and collar, which sprouts into very thin arms ending with long black claws and creamy yellow inner hand/paw pads. His slim, short legs all end with round feet dotted with black toes.
When in his mutated form, there is a major difference in his appearance. 621 is now standing at around 6 feet. His eyes are much bigger and bug out a lot more, while his ears are notably shorter than they were before mutation. His chest is quite thick and large now. Sprouting from his chest are some very slim arms that grow into large, muscular forearms ending with longer black claws. His lower torso is quite small compared to his huge chest, but these sprout into skinny legs with huge monstrous feet. On his back, his spikes have become much sharper and are 1 foot long.
621 has similar powers to Stitch, but is not as effective at using them. He is both very physical and well-equipped. He is able to use a jet pack and a grapple gun.
Basic Experiment attributes: 621 has all of the most basic abilities that many other experiments possess. His green hands and feet can produce a sticky substance that allows him to scale walls and ceilings without falling, much like a bug. He has the ability to see very well in the dark (as seen whenever his eyes turn green), lift people/objects that are many times his own weight (he isn't the strongest experiment, but is strong; one can compare his strength to that of an average human weight lifter), and his skeletal structure is so flexible that he can roll around as a ball, curled up while biting onto his legs.
Retractable limbs: 621 can retract and extract extra limbs at any time he wishes. He has an extra pair of arms that have already torn holes on the sides of his brown space suit, sharp black claws at the tip of his fingers, three porcupine-like spikes tipped with whitish-blue and black that sprout from his back, and two bug-like antennae with no real use.
DNA Mutation: An ability received after he was genetically altered by Dr. Jumba's mutator machine. His innards, thanks to the special cases of DNA he placed in the machine before mutation, have formed in a way that could boost his strength and adrenaline to alarming rates. Such change occurs upon consumption of fresh genetic material from strong, fast, very well-adaptive creatures (the very well abundant non-magical humans, unfortunately, are out of this category, as they are physically too weak for such use).
Such material comes in the form of hair, blood, or even nails. The mutation changed 621's body enough for him to handle such consumption (although, most experiments could already handle such a diet). When enough DNA is absorbed, he takes on this stronger, faster form for 5 posts. Afterwards, he reverts back to normal and will have to search for more material to consume for another mutation.
Speech: Unlike most other experiments, who usually speak Tantalog or in no language at all, 621 can also speak fluent English with a vaguely Russian accent, not unlike his creator Jumba Jookiba.
His weapons are two plasma cannons that he normally keeps hidden with his retracted extra arms. He has a secondary gun, used more for self-defense. This gun fires a beam of green energy at targets, and can teleport them short distances no more than fifteen feet. Where the target reappears is random.
621 was able to mutate himself into a much larger and stronger creature by collecting DNA strands. In this form he is physically more powerful than Stitch but less skilled.
It is unknown whether or not 621 shares Stitch's weakness of water, though it is possible, considering he is a prototype of Stitch.
- Stitch: Experiment 626, the only media where 621 makes an appearance, had its events retconned from canon in media following the game's release, though 621 could still have the same physical appearance, abilities, and personality.
- Jumba and Stitch were apprehended in Lilo & Stitch 2: Stitch Has a Glitch moments after Stitch's creation, contradicting the events chronicled in Stitch: Experiment 626. However, Jumba stated in "Kixx" that he trained Stitch between his creation and arrest, which also contradicts Lilo & Stitch 2. This would suggest that Lilo & Stitch 2 is actually non-canon or this episode is partially non-canonic.
- Even though 621 never made a physical appearance outside of Stitch: Experiment 626, his name is listed in the credits of Leroy & Stitch, suggesting in the game that 621's canon name is Chopsuey.
- If this version of Chopsuey is canon, then he is the only experiment to debut in a video game. Since the game was released on June 19, 2002, two days before Lilo & Stitch was released, he is also the one of the earliest experiments to debut.