Donmo is a friend robot to the alien protagonist Mojacko, and the two were created through the same process. As Mojacko’s robot companion, Donmo is often described as his assistant or butler, and he lives together with Mojacko and the human boy Sorao following the spaceship crash that brings them to Earth.

In terms of personality and behavior, Donmo is typically portrayed as being in a daze or a state of drowsiness. This sluggish demeanor is a central part of his character. He also has a distinct way of speaking, characterized by reversing the order of hiragana and katakana syllables during conversation. Another notable quirk is his unusual reaction to sweets; consuming too many sugary foods causes him to become drunk or unsteady.

Despite his usual lethargic state, Donmo possesses a remarkable and often crucial ability. Whenever he hits his head, he suddenly becomes highly intelligent and sharp. This transformation is a key plot device, as his sudden brilliance becomes helpful in solving various problems and difficult situations that the group encounters. His role in the story is largely defined by this dynamic, where he assists Mojacko and Sorao with their challenges, primarily during these moments of unexpected genius.

Regarding his personal motivations, Donmo has a romantic interest in a doll named Suga, who is associated with a cake shop. This crush provides a gentle, humorous subplot for his character. His key relationships are with Mojacko, to whom he is a loyal companion, and with Sorao, the human friend with whom he shares a household. There is also a noted difference between his original manga design and his anime appearance; in the anime, a line next to his mouth was removed and the color of the object on top of his head was changed.