Spooky causes chaos by fulfilling the purpose he was programmed for: terrifying his enemies by turning into their worst fears. However, he went further by directly harming his victims, aside from just scaring them, and seems to be more dangerous than most experiments. Originally malicious and slightly sadistic, he often chooses to not only provoke fear in his victims, but also cause other mental trauma such as sadness and terror. He tends to slither and hiss like a snake, despite his most common form of being a blob. When people are brave enough to stand up against his scares, Spooky becomes annoyed and apparently powerless against them. However, after being reformed, Spooky only scares people for fun on Halloween. He is also loyal and helpful to his friends, joining Stitch's rebellion to fight off Gantu.
Experiment 300 was the 300th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to terrify enemies into submission by morphing into their worst fears. 300 and the other first 624 experiments were deactivated and smuggled to Earth by Jumba during his mission to capture Experiment 626.
All of the experiment pods were released and scattered across the island of Kauai, with 300's pod landing on a house roof.
On Halloween night, a gust of wind caused 300's pod to roll down the roof's rain gutter and into a rain barrel, activating the experiment. 300 then noticed a mouse and scared it off by turning into a wild cat. When a dog attacked 300 because of the cat form, the latter frightened the dog away by turning into a dogcatcher, then fled the scene in his elastic form.
300 shortly after attacked a children's Halloween party at Lilo's hula school, scaring Mertle and the hula girls off by turning into a headless Lilo, then terrifying Moses by turning into a giant spider. Presumably after this, 300 lurked somewhere inside and waited for his next victim.
When Lilo and Stitch later arrived to discover the party in disarray, and Stitch entered the kitchen to raid the fridge, 300 locked himself in the kitchen with Stitch and almost drowned the latter by turning into floodwaters. However, 300 was forced to retreat up through an air vent when Lilo noticed his presence, relieving Stitch.
Sometime later, 300 arrived at the Pelekais' house, where he terrified Nani by turning into Cobra Bubbles and claiming he came to take Lilo away. However, when Lilo and Stitch returned to the home and noticed Cobra to be 300 from his hissing voice, they exposed 300, named Spooky, as an impostor, forcing him to hurry off again.
Lilo, Stitch, Jumba and Pleakley followed Spooky into the old mansion, where he surprised and terrorized Lilo by turning into a clown. Stitch unintentionally fell into an indoor swimming pool after Lilo's shriek startled him. Spooky then chased after Lilo in his clown form, scaring her to the point where she clung to a high pillar screaming for help.
While Lilo and Stitch were both left helpless, Spooky defeated Jumba and Pleakley by turning into their worst fears: Pleakley's mom and Jumba's ex-wife. As Stitch sank, he overcame his aquaphobia by singing "Aloha ʻOe" (a song that Lilo had taught him as a coping tactic), and after emerging from the pool, he rescued Lilo in the nick of time.
When Spooky noticed that Stitch had faced his fears, and was thus undaunted by any other scary transformations, the former turned into a bat and attempted to escape up the chimney, but was ambushed and captured in a containment orb by Stitch.
Spooky was then found a one true place scaring trick-or-treaters on Halloween, and (on other occasions) haunting the old, abandoned house.
In "The Asteroid", Spooky was one of the six experiments that Stitch convinced to come with him and Lilo to save Earth from an approaching asteroid (Spooky actually morphed into one for clarity). Along the journey, Kixx kicked the back of Spooky's seat, causing the latter (using his ability to morph into nightmarish forms) to retaliate. Spooky later attempted to use a monstrous form to scare Pleakley into giving him free soda, but Pleakley only replied with an order for Spooky to return to his seat.
The first 624 experiments, including Spooky, were rounded up by Leroy and taken to a stadium to be destroyed. However, Lilo, Stitch, Jumba, Pleakley, Reuben and Gantu arrived before the experiments could be destroyed.
Spooky participated in the following battle between the experiments and the Leroy clones by using nightmarish forms to scare Leroys.
However, the Leroys soon gained the upper hand in the battle, but were defeated when Lilo, Stitch, Reuben and several other experiments performed the song "Aloha ʻOe", which caused the Leroy army to shut down due to the original Leroy's fail-safe.
Spooky made an appearance in the Stitch! anime when Hämsterviel sent him along with Toons to scare the children of a summer camp. It is shown that he uses a form of telepathy to read his victim's mind and discover their worst fears. In "Shogun", it is shown that Spooky was dehydrated back into an experiment pod and locked in Jumba's vault.
Spooky has morphed into many forms. These include:
- A wild cat (a mouse's worst fear)
- A dogcatcher (a dog's worst fear)
- A fat, elastic, slug-shaped version of his true form
- Headless Lilo (Mertle and the hula girls' worst fear)
- A giant spider (Moses' worst fear)
- Floodwaters (Stitch's worst fear)
- Cobra Bubbles (representing Nani's worst fear of losing her sister)
- A clown (Lilo's worst fear)
- Mrs. Pleakley (twice; Pleakley's worst fear)
- Jumba's ex-wife (twice; Jumba's worst fear)
- A water monster
- A twister
- A People Eating Pest Monkey (from The Origin of Stitch
- A bat
- A giant snake (a trick-or-treater's worst fear)
- An asteroid
- A spiky, sea urchin-like fruit
- A version of his true form with massive fangs
- A large, muscular version of his true form with an alligator-like snout (a.k.a. "Spookymorp" according to Stitch's Great Escape!)
- A Goo Gobbling Booger Beast (from The Origin of Stitch)
- A zombie
- Angel (when she is angry at Stitch)
- A Tigerlily dinosaur-like creature
- Spooky was first revealed in a DVD bonus feature of Stitch! The Movie in the experiment gallery.
- Spooky bears a great resemblance to Oogie Boogie from The Nightmare Before Christmas.
- Spooky has glowing, olive-green eyes instead of black ones like most of the other experiments.
- Jumba also referred to Spooky as "X300" and mentioned that his impression of Jumba's ex-wife is frightening.
- Disney has used "X-" (X with a hyphen) in promotional material as a shorthand prefix for "Experiment".
- Unlike most experiments, Spooky's number is pronounced as a whole number ("three hundred") instead of three individual digits (which in his case would have been "three-oh-oh") as per the norm for the experiments.
- The only other canonically-seen experiment whose number is also referred to as a whole number is Woops (Experiment 600). This implies that experiments whose number is divisible by one hundred (100) are the primary exceptions to the normal by-digit pronunciation of experiment numbers.
- Spooky's pod color is green.
- Spooky is described by the experiment computer screen as, "Experiment 300. Primary function: Morphs into the victim's worst fear."
- Spooky makes a cameo in the pre-show of Stitch's Great Escape! when Gantu shows guests a quick look at the "criminals" of the galaxy (which are all experiments). However, he does not appear in his default form, but rather the large form with an alligator-like snout that he tried using on Pleakley in "The Asteroid". Underneath the image, his name is rendered as "300M/Spookymorp" (presumably meant to be "Spookymorph") in Tantalog.
- Spooky's powers and abilities resemble the Boggarts in the Harry Potter series, as they are able to turn into their victims' worst fears.