Renji Magatsu was the best friend of Yuuya Daigo. He was apart of Yuuya's group of delinquents. He would often have to deal with the abuse from his father. This never stopped until he finally decided to take his father's life.
In his delinquent days, he was a troublemaker just like Yuuya. He wasn't as bad as he seemed though. He was troubled teenage boy who lived with an abusive father. This drove him to think about murdering his father and he eventually did. He also seems to have believed in true friendship, which is what he thought he had with Yuuya. When he betrayed him, he turned vengeful.
Renji makes a return in the Martial Fist chapters. He encounters Yuuya at their old location, a run down train, and they fight only for Komaki Sakurai to stumble upon their fight. He continuously asks Yuuya why he betrayed him and this leads to an immense amount of guilt to build up inside Daigo. Not long later, he changes into a giant black snake, only for Yuuya to turn into the White Tiger also known as the Byakko.
To Yuuya Daigo: "Let's go to hell together..."