Kotaro Mochizuki (望月 コタロウ, Mochizuki Kotarō) is one of main characters of Black Blood Brothers. He is Jiro's ten-year-old, younger brother, who does not seem to exhibit powers of a vampire, such as the "Hide Hand."
He looks totally different from Jiro: Jiro is tall, has black hair and grey eyes, while he is short, has blond hair, and blue eyes.
He has the same speech pattern and bears a striking resemblance to Jiro's former lover, Alice. Later, it is revealed that Kotaro is actually the reincarnation of Alice. He has no special vampire powers because he still has not recovered the sage's memories from Jiro.
Although, he seems to have an extremely high pain tolerance, as he is thrown into the ground and walls without injury in almost every episode.
Kotaro stated that they are brothers because they are the last two remaining vampires of their special bloodline. They both carry goggles around their necks.
- As commented by Mimiko, he does not appear to be the brother of Jiro since he is not hurt by sunlight nor water, unlike Jiro.
- Kotaro's only friends were Sei's older sister the Dark Princess of the North and Grand Duke Bow-wow (a friendly grizzly bear).