Coo Erhard is a core member of the Black Gema Gema Gang, serving as its major and resident physician. Despite being only thirteen years old, he holds a medical license, having skipped grades to achieve this qualification, and his skills notably include dentistry and veterinary care. Physically, he has short dirty blonde hair and reddish-brown eyes. He possesses several canine characteristics, including a dog-like tail and a matching cap, and he occasionally exhibits corresponding behaviors, such as leaping to catch airborne food in his mouth. He is frequently seen holding a stuffed panda, which serves as a cherished memento.

Coo shares a deep and long-standing bond with Piyoko, the eight-year-old leader of the gang, having been her childhood friend and now acting as her closest confidant. Within the group's self-perceived family structure, Coo assumes the role of the older brother, a dynamic that holds even though Piyoko mistakenly believes herself to be his senior. His primary function within the gang is as a member of the PKO unit, alongside Rik Heisenberg and Ky Schweitzer, where the trio is tasked with Piyoko's protection and the execution of her schemes. In this capacity, he often acts as the group's pragmatic and skeptical voice, routinely voicing doubts about Piyoko's plans when they appear ill-conceived or unworkable.

Personality-wise, Coo generally presents a calm and level-headed demeanor, displaying adaptability in both leadership and social situations. However, this composure can give way to moments of brattiness, childishness, or defiance, particularly when he feels he is being bossed around or if his medical competence is questioned due to his age. His loyalty to Piyoko is unwavering, creating a relaxed dynamic between them that softens the group's hierarchy and motivates his actions, which are fundamentally geared toward supporting her objectives, even as he mediates between her grand schemes and the practical duties of their subordinates. While generally dismissive of others, he exhibits a notable shift in character when performing serious medical duties, becoming more formal, apathetic, and focused to prevent mistakes.

The narrative role of Coo in A Trip to the Planet is primarily as a member of the antagonistic force fixated on the protagonist Dejiko, often through kidnapping-for-ransom plots. He and his fellow PKO members function as one of several primary caretakers for Piyoko, undertaking missions that typically result in comedic setbacks or distractions. His expertise is demonstrated through collaborations with his comrades, fulfilling his role not just as a doctor, but as a key operational support for the gang's often flawed endeavors.