I should probably start off by telling you our main protagonist is Issei hyoudou's older brother only about a few months older though. His name is Hisashi Kawasaki, Hisashi is actually Issei's step brother since they had different moms but the same dad but they act like they are normal brothers. Due to his real mom missing Hisashi doesn't really enjoy his life. One day when Hisashi was at school being embarrassed by Issei's perverting, Hisashi was walking home from school when he noticed the crimson red hair of Rias Gremory and was dazzled by the view of her sitting on the door step at his home. Once Hisashi asked her why she was waiting at his family's house she replied "waiting for you and Issei". After some confusion Rias took Hisashi back to the school grounds and went into the old school building. She then showed Hisashi that she was a devil and that she wants to turn Hisashi and Issei into a devils servant. Once Hisashi realised this could be a new life for him, he accepted the pawn chess piece and put the piece up to his chest to confirm Hisashi is ready and the chess piece started to go inside of his body. Hisashi is now a new born devil, all that's left is Issei. Rias also told Hisashi that he and Issei have powers known as the Sacred Gear.

All of a sudden Rias told Hisashi that a fallen angel is about to attack Issei on their date, before Hisashi realised what Rias just said a portal opened using Rias's power and they found themselves at a park in town. Hisashi turned around to see Issei on the floor covered with blood. Hisashi didn't know how to handle this until he noticed the fallen angle flying away. "Ill kill her, I will kill that BITCH". Rias told him to calm down and that Rias would revive him as a devil so don't worry.

The next day Issei was asking his friends if they remember his girlfriend, yet no one remembers her. At this point Hisashi was taking the day off so he could talk to Rias and Akeno in the old school building. He was wondering about his sacred gear and how to unlock this power. Once Hisashi asked them Rias replied "raise your arm you are best suited with then concentrate on the strongest part of your body". Hisashi wasn't sure which part of him was strong but then he realised when he was a kid he would always fight the other kids due to Hisashi being angry all the time, and he would always knock out all the other kids with one punch with his right hand. Once realising this Hisashi put his right arm up and focused on the memory of those fights. Then this hand started to glow black, a second later an enormous amount of energy came off the sacred gear Rias and Akeno realised this straight away happy but a bit scared. Then the Sacred Gear showed up on Hisashi's right hand. Rias explained to Hisashi that his sacred gear at it's current state gives Hisashi 3x the strength of a body builder.

Later that night Hisashi went over to Issei's friends house to see how he was doing. Once talking to Issei's friends they said he left feeling ill. So Hisashi decided to look for him, 10 minutes later he notice Issei is in the park where the fallen angel he was dating killed him. Hisashi turned about 90 degrees to see another fallen angel about to kill Issei. Before Hisashi had time to react the fallen angel put a spear of light through Issei's stomach. Hisashi immediately jumped out in front of the fallen angle activating his sacred gear and gives a perfect blow to the fallen angle's head, knocking him to the ground. Rias, Akeno and Koneko showed up through a teleportation circle and noticed Issei on the floor almost dead…again. Rias calmed Hisashi down while the fallen angel escapes and takes Issei back to his house so Rias can heal him.