Because Swapper is two heads on the same body, Swapper is two beings cooperating as one, though their personalities mirror each other: they can be indecisive at times but usually work well together. An example is when Stitch in Lilo's body asked the two-headed cousin to switch their minds back, Swapper, after hesitating, blew a raspberry in response. The two enjoy messing with people by swapping their brains and refuse to switch them back unless threatened. However, Swapper will eventually switch individuals' minds back into their original bodies when both heads choose to do so.
Experiment 355 was the 355th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to swap any two (or more) individuals' minds. According to Jumba, 355 once swapped the former and his wife for an entire month, which led to a divorce in the end. 355 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.
At an unknown point after this, Experiment 355 was activated and captured by Gantu.
When Gantu was about to send 355 to Hämsterviel, 355 got loose and swapped Gantu and Hämsterviel's minds, allowing 355 to escape.
Hämsterviel in Gantu's body then decided to go AWOL and recapture 355, but refused to have 355 switch their minds back afterwards, although Gantu in Hämsterviel's body protested.
After 355, named Swapper, refused to switch Lilo and Stitch's minds back, they took him back to their home, where Jumba covered up Swapper's container with a towel to prevent the latter from using his mind-swapping power on the former.
When Pleakley removed the towel, Swapper switched his and Jumba's minds as well, forcing Lilo's dysfunctional ʻohana to throw a "Lilo's Not Weird" party that she hosted earlier, after Victoria, Mertle and the hula girls showed up.
Later, Gantu with Hämsterviel's mind arrived at the Pelekais' house for Swapper, where a fight ensued during which Swapper switched Lilo, Stitch, Jumba and Pleakley's minds again, though each was in someone else's body.
After Hämsterviel in Gantu's body was defeated, Swapper switched Lilo, Stitch, Jumba and Pleakley's minds back into their original bodies, and was then forced to switch Gantu and Hämsterviel's minds back as well under the threat of being given Pleakley's disgusting cuisine for dinner.
The first 624 experiments, including Swapper, 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.
Swapper participated in the following battle between the experiments and the Leroy clones.
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. Swapper joins them for the song by playing the keyboard.
Swapper made an appearance in the Stitch! anime where he used his powers on Stitch and Hiroman. Until they could find Swapper again, Hiroman needed to figure out how to use Stitch's abilities to take on Heat. After Heat, Gantu and Reuben were defeated, Jumba made a request for Swapper to swap Stitch and Hiroman back into their original bodies, but he instead swapped everyone's bodies again!
- Swapper's pod color is blue.
- Swapper is described by the experiment computer screen as, "Experiment 355. Primary function: Swaps people's minds."