Son of the woman who Tsubasa's father is dating. He's a member, along with the minor characters Martin, Joker, Ushio and Atsuya, of an indie-rock band (called "Yin & Yang") Because of his bleached hair and skull jewelry, he is initially looked upon with suspicion by Tsubasa. Because he wants his mother to be happy and eager to have a little sister, he has no objections to their marriage. However, he makes the mistake of assuming that Tsubasa is an elementary school student due to her short stature, causing Tsubasa to have a spectacular meltdown. (Tsubasa is in fact, one month older than Kazuma.)

Spoiler

Kazuma's most valued treasures are Tsubasa and his music. He slowly comes to terms with the fact that his love for Tsubasa is so much more than brotherly affection. And also with the fact that music will eventually drive him apart from her...