Rimu Mikogami, one of the Mikogami twins residing in Kamihama, possesses indigo hair and blue eyes typically obscured by large vision-correcting glasses; their removal severely impairs her functionality. Alongside her sisters, she discovers a Kirumin compact in their attic that unlocks her ability to transform into animals. Her primary Kirumin form manifests as a green-associated rabbit, granting heightened hearing and accelerated speed in Costume Mode. Subsequent Kirumin effect evolutions expand her transformations to include creatures like the platypus.

As an elementary school reporter, she documents events with an apple-shaped camera and notepad. Rimu exhibits a reserved, logical demeanor, serving as her group's voice of reason. Initially reliant on her energetic twin Riko for initiative, she gradually cultivates self-assurance. Her methodical observations prove instrumental in the Detective Club's investigations of animal-related mysteries.

A pivotal revelation discloses her one-quarter Animalian heritage, enabling natural transformation without the Kirumin device. This lineage surfaces late in the series through spontaneous bat metamorphosis—a trait shared with her cousin Kanon Hatori. It also explains her latent animal communication skill, which she refines under her mother's guidance.

Her relationships include an unspoken romantic tension with Tamao Kijima, humorously marked by his glasses cracking when Rimu emotionally impacts him. She maintains close bonds with sisters Riko and Nagisa, friend Ken Inomata, and reconciles with initially antagonistic Kanon upon uncovering their familial connection.

Rimu confronts physical trials like navigating booby traps without glasses and emotional growth through team reliance during crises such as cave rescues. Her Animalian abilities culminate in the finale, affirming permanent bat transformation independent of the Kirumin device, resonating with the series' human-animal identity themes.