Ga-Rei -Zero- is set as the prequel to Ga-Rei the manga. The manga, itself, is a supernatural action shounen so Ga-Rei -Zero- is set in the same way. While knowing the setting of the manga helps to understand the anime better, it is definitely not required to read the manga beforehand to enjoy Ga-Rei -Zero-. This viewer had never heard of Ga-Rei before the anime, yet was still able to enjoy the Ga-Rei -Zero-.
For those not familiar with the Ga-Rei setting, it is essentially about an agency that hunts rogue spirits using exorcists. A Ga-Rei is a spirit beast that exorcists use in addition to other exotic weapons and traditional swords to fight supernatural enemies.
Story: The storytelling for Ga-Rei was nothing short of amazing. The first two episodes occur chronologically towards the end of the series, but set up a conflict during which the rest of the series tries to explain how events lead up to such a conflict. This leaves the viewer constantly guessing throughout the series, giving enjoyment through suspense. At the same time, the viewer is greeted with several humorous scenes through the show that help lighten the mood and make you enjoy the characters.
Art: The animation for Ga-Rei isn't extraordinary or anything. It gets the job done. The supernatural enemies look disgusting and allow you to develop a hatred for them. The characters are drawn in a way that you develop interest in them. The action looks fluid and is pleasing to watch.
Sound: What really stood out to me in Ga-Rei was the sound. The background music really fit the mood well and after every episode, it left me questioning when the OST would come out. Furthermore, the Ga-Rei anime comes out with an amazing image and insert song CD, many of the songs which are used in fitting moments throughout Ga-Rei. Both the OP and ED fit the anime as well.
Character: Ga-Rei really focuses on developing the two main characters, Kagura and Yomi. While I feel that the other characters weren't developed as much as I would have liked, the story is really focused on Kagura and Yomi and only their development is crucial to the story. The way Kagura and Yomi interact together and towards other characters seems appropriate for their age and their past experiences and because of their interaction with each other, they change and part of the story is watching their growth.
Enjoyment: The combination of a great storyline as well as well-done action sequences put to an outstanding soundtrack makes this anime incredibly enjoyable for me. Once you start the anime, at least for the first couple of episodes, it's almost impossible to stop. The first few episodes demand the viewer to watch them one after another. Combined with the humorous yuri moments and other cute moments, Ga-Rei does its best to tell a story that can both standalone, but at the same time, persuade the viewer to continue the series into the manga.