Renji Magatsu is a childhood friend of Yuuya Daigo. He was a quiet, withdrawn, and sickly teenager, always being physically abused by his drunkard father. Renji grew strongly attached to "Yuu-chan" because Daigo was his complete opposite, strong both in body and mind, and yet understood him; sympathetic Yuuya was always willing to listen and support Renji.
The two friends made a pact in middle school to kill their fathers, but in the end, Yuuya couldn't murder his parent because he discovered his resentment was ill-founded. Renji, on the other hand, had no troubles stabbing his father to death, and Yuuya was too late to stop him. When police arrived and Renji realized that Yuuya hadn't upheld his end, he felt entirely betrayed. Imprisoned in the Kanto Area Juvenile Corrections and Treatment Facility, Renji held fast to his hatred for Yuuya Daigo... and then Munetaka Yagyuu offered him a chance at revenge.Renji Magatsu became a member of the Twelve Generals, and as the Martial Fist was set to assassinate the Magami Five, he naturally went against Yuuya Daigo, drawing his old friend back to the place they used to hang out. With Renji calling him a traitor, Yuuya couldn't fight him, instead running away.
So Renji uses Komaki Sakurai to lure Yuuya to fight, showing that he has supernatural powers, too; Renji can turn anything and everything to stone. With Komaki threatened, Yuuya metamorphes into Byakko, the Deity awakening inside the Vessel, and Renji is transformed into a snake demon by the Dark Arts.