Karly Miller, originally named Kodama Morino, is the youngest quintuplet in the Miller family, born minutes after her siblings and holding a distinct position in their dynamic. She exhibits a passionate dedication to animal welfare and environmental causes, explicitly aiming to save the world and protect all animals—except frogs, which she consistently avoids. Her energetic and optimistic nature drives her to initiate or join adventures alongside her siblings. She balances curiosity and empathy, particularly toward animals and nature. While cooperative within the quintuplet group, she pursues individual goals that spark storylines centered on animal rescues or environmental initiatives.

One analysis portrays her as enthusiastic and imaginative, drawn to new experiences while upholding moral principles. This interpretation notes she relies on trusted role models to shape her behavior and strives to positively impact her surroundings, though she may show indecision when faced with multiple possibilities—overcoming it through adaptability. An alternate source classifies her as an ISFP personality type, without elaborating on the reasoning.

Within the family, she displays a protective instinct and loyalty toward her siblings during group challenges. Episodes like "Karly's Un-Birthday" and "Karly's Frog Adventure" spotlight how her animal advocacy and anxieties propel plots, manifesting her aspirations into concrete actions despite complications from her amphibian aversion. No documented spin-offs, movies, or OVAs expand her character development across additional media formats.