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Yuki grew up in a loving and wealthy family. His parents have never had any problem with his choice to become a musician, as they agreed to fund whatever choice he made and like to throw their money around and feel important. In school, Yuki was never interested in getting good grades and spent most of his time getting into trouble with Hiro, who was his best friend in high school. His younger brother is much more academic, but their parents pay more attention to their eldest son. As a result, Yuki is spoiled and always insists on having his own way, no matter what.
He became the third band member, after a chance meeting in the street, with Akemi and Karin. Akemi felt that Yuki's voice was perfect for his music and offered him the position as vocalist straight away.
Some time before the beginning of the story, Yuki saw a picture of Rio in a magazine. He became an obsessive fan and made it a goal in life to have sex with him one day. His dream becomes a reality later in the story (after a few years of fund-raising) and he soon uses his powers of "I always get what I want" to make Rio his 'official' boyfriend (much to the dismay of Zora, who has had feelings for Rio for a while, by this point).

Yuki spends a lot of his free time with Hiro, at his apartment (drinking, smoking, playing video games, generally being lazy and not doing much of anything productive). He also spends a lot of time with Akemi, working on music and lyrics. He gets along well with Karin and Shouji, but isn't as close to either of them. He has a fairly awkward relationship with Zora (who is in love with his boyfriend, Rio) and Yoshi (who is in love with his best friend, Hiro). He doesn't spend a huge amount of time with Rio outside of the bedroom (although they spend a lot of time there).
He later has a brief sexual encounter with Toshio, who is infatuated with him, but doesn't see it as anything serious. He and Hiro accept Soshi as a good friend later, and include him in their drinking-and-not-doing-much sessions, as his personality fits in well with them.