As a human child he was abandoned in a manhole and never knew anything about his origin, his parents or where he had come from. Because he lived in the sewers, Kou, in his naivety would always dream of obtaining the blue sky he saw above. One day, some soldiers looking for rebels arrived as Kou was above ground and randomly shot a pair of mother and child in front of him as an example to potential rebels. Kou didn’t understand what was going, much less about what the solders that found him intended to do to him when they saw his face and sent him to the orphanage and upon arriving there and being able to have good food, clean clothes and all the nice things he wasn’t able to have in the sewers, Kou thought of the orphanage as heaven.
Then Kou was taken to a ‘club’. Apparently because of a war that was going on, the country needed a lot of money and so to earn quick money to fund the war, they chose beautiful children from the orphanage like Kou and offered them up as ‘merchandise’ for the aristocracy’ and rich people’s ‘entertainment’. Kou ended up whipped, cut, beaten up, tortured and apparently he was so ‘popular’ and in demand that he didn’t even have time to heal any of his bruises or wounds. Kou didn't want to think that this was ‘painful’ and wanted to believe that he should be happy despite this because they let him have nice things. He didn’t have to sleep in a stinky place, and no longer he had to dig through trash for scraps. Eventually he accepted that the situation he was in was basically hell, so in attempt to stop the orphanage from making him continue going to be the aristocrats’ toy, he lamented that his right eye might never see the blue sky he craved again. He decided to disfigure himself by gouging out his right eye. But unfortunately, that only made the twisted aristocrats desire him more. Kou realized that he made a mistake and he was fearful that his situation would only get worse and he’ll have even more of his freedom stripped away. Kou tried to commit suicide - but found himself unable to do it. This was when he met Ruki, who called Kou a loser. Kou denied that and tried to kill himself - Ruki even dared him to - but again, Kou was unable to do it, which is basically how Ruki expected to go. Ruki told Kou that the reason Kou couldn’t do it despite desperately wanting to escape his situation was because Kou still had ‘hope’. Ruki knew this because he often catched Kou staring at the sky. Ruki told Kou that if he gave up that hope, he’ll be nothing more than a loser, before inviting Kou to come with him if he wanted to be free - whether it be freedom, or the blue sky, he’ll be able to have anything he wanted, free from this hell. So Ruki, Kou, Yuma became friends and accompliced and plotted their escape - which was initially successful but then the orphanage people catched up to them and Kou was shot and injured multiple times - the pursuers citing that it didn’t matter if they hurt the ‘merchandise’ because Kou’s ‘patrons’ were the sick type that liked him the more damaged he is. But they ended up going too far, and Kou was pretty much near death when he was brought back to the orphanage. As Kou’s life was fading and he lamented that he didn’t want to die, not when he still wasn’t able to obtain the blue sky that he desired, Karlheinz appeared with the offer to make him a vampire.
After that the Mukami's were taken in by Karlheinz, he even gave Kou a new magic ‘eye’, one that’s fragile and seems like a jewel but also allows Kou to see the truth by reading the heart of humans. Karlheinz told him that it was because he knew that aside from him and Kou’s brothers, Kou has trouble believing in other people, much less humans due to his past experiences up to that point. However, Karlheinz also warned Kou not to rely on it too much and lose sight of what’s really important to him. He also adds that when Kou’s matched with a certain blood and understands a certain truth, the eye would show him something that’s like a miracle/wonder. Karlheinz took them in and prepare them to help him with the Adam and Eve thing, where there’d be a human girl with the heart of the demon lord’s daughter, and Eve’s blood would choose an “Adam” who will be able to obtain the power to control the world and together their descendants would change the world. While Kou doesn’t really get it, much less the conditions of making it happen, just like the rest of his brothers he wants to help fulfill their benefactor’s wish.