Mongar, also known as Monga or Monger-chan, is a diminutive extraterrestrial who serves as a steadfast companion to the protagonist throughout multiple storylines. His full designation, Monger-chan, underscores his alien nature and small stature.

As an alien life form, Mongar displays distinctive physiological traits. His emotions manifest through color changes, with happiness turning his body yellow. His capabilities include functioning as an extreme omnivore, able to consume virtually any substance, a trait that drives both practical utility and narrative complications. Despite his non-human biology, Mongar achieves basic communication with humans, primarily using simplified Japanese.

Mongar's core narrative function involves assisting the main character in managing the family's interstellar hotel and navigating unforeseen events. This partnership entails supporting daily hospitality duties while also participating in unexpected adventures that frequently venture beyond Earth. His dual role bridges traditional family legacy with futuristic space exploration themes.

His supportive role and fundamental characteristics remain consistent across all adaptations, including the 1981 film where he forms a core trio with the protagonist and robotic companion Gonsuke. This consistency persists throughout the television series and subsequent films. Mongar's teleportation ability enables instantaneous travel across cosmic distances, facilitating seamless shifts between terrestrial hotel operations and extraterrestrial exploration, cementing his role in both settings.