Guarda Streamingcommunity. Sin is a rock star. He has an amazingly loud voice with a range of four octaves. His secret is that he has created his voice through a method of vocal cord doping. His voice is now teetering on the edge, due to side effects from the vocal cord doping. Around this time, Sin meets Fūka, who is a street musician. When Fūka sings, her voice is so small. Due to Fūka, Sin looks back upon his past.