Yaarp is basically very friendly, but responds quickly and dramatically to the way others treat him. For example, if he feels intimidated or provoked, he will emit loud sonic blasts. If he feels safe or unthreatened, he will emit small, gentle honks as a sign of affection and friendship.
Yaarp, for the most part, is pretty easygoing and laid-back. However, he is very ready-minded and cautious, but is loyal to those who win his trust. He is also a bit jumpy and nervous, often sounding his horn at improper times because something trivial had startled him.
Experiment 613 was the 613th experiment to be created by Jumba with Hämsterviel's funding. He was designed to use his sonic-blasting ability to shatter buildings and deafen nearby victims. 613 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 613's pod landing in Mrs. Hasagawa's fruit.
When Mrs. Hasagawa watered her fruit, 613's pod got wet, activating the experiment. After being admired by a couple on their honeymoon, 613 then caused massive chaos using his destructive shock waves.
When 613 was later chased into an electronics store by Gantu wearing special earmuffs, the former was captured in a container, but rescued and freed by Stitch. Instead of showing gratitude, 613 emitted a sonic blast that made Stitch go temporarily deaf, and then fled the store with Gantu in hot pursuit.
Since Stitch was highly sensitive to 613's shock waves, Pleakley was chosen for the task. After Lilo, Stitch and Jumba attempted to train and condition Pleakley, he was equipped with experiment catching gear. They tracked 613 down to a cave, where Stitch, using a conch, posed as the experiment and acted like a decoy. He then imitated 613's sonic blast to alert Gantu, who donned his earmuffs and pursued Stitch.
Meanwhile, a reluctant Pleakley entered the cave and discovered 613. When Pleakley unintentionally blasted 613 with a vacuum, the latter retaliated using a massive shock wave. However, when a sudden cave-in endangered 613's life, he was then saved by Pleakley. 613 was grateful and kissed Pleakley, who christened him Yaarp.
Pleakley and Yaarp then fled the crumbling cave and made it out safely. When Yaarp first noticed Lilo, he growled until Pleakley assured him that she was a friend, causing Yaarp to emit a gentle honk. Unfortunately, Stitch got captured by Gantu (who thought Stitch was actually Yaarp) during the process.
As part of a setup, Yaarp was taken to the local museum for a ransom exchange. When Lilo arrived with Yaarp in a pet carrier, the former made Gantu believe that Yaarp was rehabilitated. However, (as planned) when Gantu removed his earmuffs and bartered Stitch for Yaarp, the latter unleashed a sonic blast upon Lilo's signal, alerting the authorities and forcing Gantu to flee.
Yaarp reappeared in "Elastico" as one of the audience members for Lilo's hula dance.
Yaarp reappeared in "Spike" as the scorekeeper for the Ohana-Rama, and was also briefly seen hiding under the couch during Pleakley's E.A.R.W.A.X. session.
In "Remmy", Yaarp was one of the experiments in Lilo's dream.
In the film's opening Yaarp can be seen greeting Lilo with sonic blasts from his megaphone-like antenna.
Later, the first 624 experiments, including Yaarp, 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.
Yaarp 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. Yaarp joins them for the song by playing the built-in foghorn on his head.
- Yaarp was first revealed in a DVD bonus feature of Stitch! The Movie in the experiment gallery.
- Yaarp is one of the ten known experiments named by someone other than Lilo, the other nine being Gigi, Elastico, Hammerface, Heat, Thresher, Plasmoid, Morpholomew, Shrink, and Leroy.
- Yaarp was named after a word from Pleakley's home planet.
- Yaarp's pod color is yellow.
- Yaarp is described by the experiment computer screen as, "Experiment 613. Primary function: Noisemaker."
- In "Spike", only Spike, Fibber, Nosy, Bonnie, Clyde, Slushy and Splodyhead were part of Pleakley's E.A.R.W.A.X. group, but in one scene, Yaarp can clearly be seen hiding under the couch.
- Yaarp's torso and tail appear in the online game Jumba's Lab.