Younger brother of Jirou Mochizuki, he does not seem to exhibit powers of a vampire, such as the "Hide Hand." Although, he seems to have an extremely high pain tolerancy: as he is thrown into the ground and walls without injury in almost every episode. As commented by Mimiko Katsuragi, he does not appear to be the brother of Jirou since he is not hurt by sunlight nor water, unlike Jirou. Another fact is that they look totally different from each other: Jirou is tall, has black hair and brown eyes, while his brother is short, has blond hair, and blue eyes. Kotarou stated that they are brothers because they are the last two remaining vampires of their special bloodline. They both carry goggles around their necks. He has the same speech pattern and bears a striking resemblance to Jirou's former lover, Alice.
Later, it is revealed that Kotarou Mochizuki is actually the reincarnation of Alice.