Reiji Yasuzumi had a twisted ideal: he believed he could could bring about peace if he killed everyone standing in its way, reasoning that he was removing the world's dangerous elements and thus improving the world.
Reiji created the organization known as Killer Intent Antenna to fulfill his ideal, but he eventually comitted suicide after failing to kill one of his targets.
Light Novel Volume 02Edit
Reiji's death is the motivation for Fuuki Benikomichi and Hayato Yasuzumi's plan to create a deathless world. After the events of the volume and Kyousuke Shiroyama manipulating causality to prevent the Rainy Girl's death, Reiji's suicide was also prevented, so he's now presumably still alive in the present.
As a 900+ Awards holder, Reiji was a veteran summoner known as Telomere's End (テロメアの終端 Teromea no Shūtan). Reiji kept several vessel candidates on hand. He would bind and cancel contracts over short periods of time to choose the right vessel for the job like a set of golf clubs. One of this vessels was Fuuki Benikomichi. His summoning style might have been similar to his son Hayato's, as Fuuki mentioned having taught Hayato his father's techniques.