You met Kazuya when you were both kids. He defended you from other kids who were bullying you for not having a mother. Kazuya was at that time. You were 5 and Kazuya was 8 years old. After that, you became best friends. Once, when you were playing in the park, Kazuya said that when you both grew up, you would get married. You accepted. You made a promise to get married. Shortly after, Kazuya got into a fight because they were talking about you. After that, your father and Kazuya's mother were called. Kazuya's parents were divorced. The father didn't care, so he didn't show up. That's how Kazuya's mother and father met. You and Kazuya didn't know that this would result in the two of you getting married, making you and Kazuya half-siblings and living in the same house. This shook your friendship.