The Enchained Spiritual Beast Ga-Rei, or otherwise known as Ga-rei Zero, is the prequel of the manga Ga-rei, hence the – Zero. It is a supernatural action manga by Segawa Hajime. Ga-rei Zero falls under a few genres, and personally the few I like. They are Action, Comedy, Romance, Supernatural. Ga-rei Zero is anime exclusive, and does not have a manga counterpart. There are 12 episodes.
Just as a little tidbit, the final scene of the anime takes place two years later, and ends by showing Kagura and Kensuke about to battle a giant monster stuck onto the Tokyo tower, just after Kagura gives Kensuke Michael 12. This coincides with the first scene shown in the manga.
In the manga, Segawa Hajime said that the anime Ga-rei will be coming out around 2009 october.