Arnold is one of the three mouse friends that the protagonist Topo Gigio meets upon arriving on Earth. He is part of a trio that includes Kurt and Pierre, and together they are the first characters to befriend the time-displaced astronaut mouse. Arnold, along with his two companions, lives in the coastal town of Catalina and is a supporting character in the series, providing friendship and companionship to Topo Gigio as he adjusts to life in the past. No specific information regarding his individual personality, unique motivations, personal development throughout the story, notable abilities, or distinct role beyond being a member of this friendly mouse trio is available. His key relationships are defined by his close friendship with Kurt and Pierre, as well as his supportive association with Topo Gigio. The character of Arnold is a consistent presence as part of the group of local mice who accept the protagonist into their world.