Experiment 025 was the 25th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to use his bright light to pose as a beacon for an invasion to the Galactic Armada. 025 and the other first 624 experiments were deactivated and smuggled to Earth by Jumba during his mission to capture Experiment 626.
Meanwhile, Lilo was attempting to teach Stitch about Christmas when the latter suddenly noticed a woman putting 025's pod in a gift box. Stitch hastily grabbed what he thought was the giftwrapped pod, but when he tried to tell Lilo, she ignored him.
When Gantu later came by to pick up 025's pod, the same woman who had wrapped it earlier hinted that there was a mishap involving Stitch, much to Gantu's anger. He then swore to avenge Stitch for stealing Hämsterviel's gift.
In an attempt to find the pod, Stitch stole countless presents from every house and the local mall. Lilo assumed Stitch was selfishly hoarding gifts for himself, but after the latter was captured and interrogated, he revealed what he was really trying to do. Shortly after, Gantu attacked the Pelekai residence and managed to make off with all the presents that Stitch had stolen beforehand.
When Lilo, Stitch, Jumba and Pleakley located the experiment pod in town, the four tussled with Gantu dressed in a Santa outfit. Lilo and Stitch snatched the giftwrapped pod from Gantu, although he retrieved it after a long chase. However, when Gantu attempted to crush Lilo for meddling, she reprimanded him until he admitted that he intended to give 025's pod to Hämsterviel as a Christmas present.
Just then, a little girl asked Gantu for a mere Christmas gift, believing he was actually Santa. Forced to comply, Gantu ended up giving 025's pod to her and later distributed presents to other children, thereby honoring the true spirit of Christmas.
After Jumba discovered 025 to be harmless, Lilo activated 025 in a fountain. She then named him Topper, shared him with the little girl seen from before, and gave him a one true place atop the local Christmas tree (actually a decorated Pleakley) as a Christmas star, while Topper's bright light unintentionally signaled the Galactic Armada.
Topper reappeared in "Elastico" as one of the audience members for Lilo's hula dance.
Topper was one of the experiments that was gliding on the screen, but he is 591 instead of 025.
The first 624 experiments, including Topper, 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.
Topper participated in the following battle between the experiments and the Leroy clones, but it is unknown what he did. However, it is presumed that he wouldn't have been able to be very effective, due to the fact that his powers are deemed useless.
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.
- Topper was first revealed in a DVD bonus feature of Stitch! The Movie in the experiment gallery.
- Topper's primary function is to use his bright light to signal the alien attack fleet. However, on the Disney site, it is incorrectly stated that Topper's light is designed to keep people awake.
- Even though Topper was found his one true place on a Christmas tree, it is unknown what he does during the other seasons.
- Topper's pod color is white.
- Topper is described by the experiment computer screen as, "Experiment 025. Primary function: Keeper of awakeness to people."
- His number is a likely reference to Christmas Day (December 25).