Ayakashi

Ayakashi is a parasite life force that gives superpowers to its host, taking in return the host's entire life force. Yu Kusaka is a student that lost his will to live after the death of a dear childhood friend, until the day a mysterious girl, named Eim Yoake, appears. From that moment on, his life would never be the same again. Hunted by Ayakashi and their hosts, the power within Yu awakens and a never ending battle begins.

(Source: Wikipedia)