Coo Erhard holds the rank of Major within the Black Gema Gema Corps, first appearing in the Di Gi Charat Christmas Special. Despite his apparent youth, he serves as a competent physician.

Physically, Coo possesses a dog tail, consistent with other PKO unit members. He wears a distinctive dog cap and has short, dirty blonde hair paired with reddish-brown eyes.

Personality-wise, Coo displays selfish or bratty tendencies, especially when receiving orders due to his dislike of being bossed around. His general demeanor, however, is described as quite chill.