Coo Erhard is the 13-year-old major of the Black Gema Gema Gang, dedicated to supporting leader Piyoko's mission to kidnap Dejiko. Despite his youth, he demonstrates notable competence as a physician. His canine traits include a visible dog tail and he typically wears a dog cap, sporting short, dirty blonde hair and reddish-brown eyes. Coo generally presents a calm, easygoing demeanor but can display selfish or bratty tendencies, especially when resisting authority. His loyalty to Piyoko is unwavering, marked by romantic feelings that manifest in his role as her self-designated "pet." As the gang's youngest core member, he operates alongside Ky Schweitzer, the 17-year-old dentist acting as Piyoko's "mother," and Rik Heisenberg, the 26-year-old veterinarian acting as her "father." Together, they execute schemes involving interdimensional travel, like pursuing Dejiko to America during the Summer Vacation Special.