Rustblaster

Rustblaster (ラストブラスター) is a manga series by Yana Toboso. It was serialized in GFantasy.

Plot
In the Vampire Mercenary Specialized School, "1000 Years Academy," the most unvampire-like vampire at the academy, Aru, and the human transfer student, Kei, meet by chance. But what they didn't know was that it was a fate the world depended on.