Lassie, a dog owned by Rei Ichidō, takes its name from the iconic television character of the same title. The character displays anthropomorphic traits such as walking upright and mimicking human behaviors, reinforcing the series' comedic absurdity. Acting as Rei's eccentric counterpart, Lassie functions as a comedic sidekick, engaging in the group's surreal antics. Its design and mannerisms employ "Funny Animal" tropes to amplify the narrative's surreal humor.

In pivotal episodes, Lassie drives key plotlines. "The Tale of Loyal Dog Lassie" centers on a dramatic rescue mission, embodying the loyalty archetype referenced in its title. "Worry Explosion: Lassie Becomes a Dad" explores themes of unexpected parenthood as Lassie fathers puppies, weaving responsibility into the absurdity. These storylines cement Lassie’s role within the core cast’s escapades while preserving its anthropomorphic comedy.

Lassie possesses the Reality Warper ability shared among the Kimengumi, leveraging Toon Physics for exaggerated gags. This includes defying natural laws through elastic transformations, aligning with the series' episodic gag structure. During such comedic sequences, Lassie frequently adopts a Super-Deformed (chibi) appearance. The character retains its original name in localized adaptations like the French dub, contrasting with human characters who receive Westernized aliases.