BlazBlue: Phase Shift 1

BlazBlue: Phase Shift 1 is a light novel written by Mako Komao and published by Fujimi Shobo. It was released on April 2011.

Description
The story revolves around Kazuma, a young student of the Magic guild that looks like Hazama. Celica apparently dislikes him and feels uncomfortable being around him. It seems that he's actually not human, and was created by Relius Clover to act as a new body for Yūki Terumi (who it seems loses his previous body whenever the Black Beast appears and vanishes). Terumi doesn't take over the body, rather he needs to form a mutual pact with Kazuma to fuse with him.

To try and stop that, Mitsuyoshi's younger brother, Tomonori, goes after Kazuma to kill him. He arrives too late and Terumi has already obtained the body. Terumi uses Ouroboros to kill Tomonori, but then gets defeated by Mitsuyoshi and Valkenhayn, and is sealed away by Clavis Alucard until the Dark War, when Konoe disregards the potential danger and unseals him. She uses a restricting compulsion spell on him called "Ruby: Mind Eater'" that basically makes him a puppet that has to follow her orders and help them defeat the Black Beast.