Kiseijuu: Sei no Kakuritsu

One day, high school boy Shinichi wakes up to find that his hand has been possesses by a talking alien parasite Although he and his parasite are able to form a reluctant symbiotic relationship of sorts, the other parasites on Earth have taken over their hosts' bodies completely. Moreover, there are brutal murders happening all across the country. And while the police are baffled as they look for a killer, Shinichi is the only one who knows the truth: It's not a serial killer. It's the parasites feeding on human flesh.