Hina is Rui's biological elder sister, Natsuo's older step-sister and his first love interest. They meet at the rooftop of the school where she has just started working as an English teacher. As they continue running into each other there, he notices that it only happens when she is conflicted at something, for which he offers his sincere support. Such kindness plus his defined goal of becoming a writer makes a deep impression on her, which in time leads her into falling for him. The fact of their parents' remarriage, their starting living together, and his finding of her unworthy relationship with her married former sensei, finally made her resolve to put in order her life, just time before she discovers her younger sister is becoming fond of him. After trying so hard to push him aside, she can't help getting jealous, which ends up into opening her heart to him and starting a forbidden relationship.
After dating with Rui, their relationship becomes to seemingly normal sister and brother as Natsuo was unaware that Hina was still keeping her feelings after their break up. Natsuo seemed to care for her almost as much as when they were a couple, proven when he even shielded Hina from Kengo Tanabe's attempt to stab her.
Rui is Hina's biological younger sister, Natsuo's younger step-sister and his latest former lover. They meet at a mixer set up by classmates of their respective schools. While having fizzy drinks out of the karaoke booth, she tells him to sneak out. So, taking him to her house, she proposes him to have their first sexual experience together, which is supposed to be just a one-night-stand.
After their parents' remarriage and since they all start living together in the same house, she steadily develops feelings towards him. Unknowingly, it is her resolve on trying to seduce him that backfires because it leads him into starting a relationship with her elder sister.
After he breaks up with Hina, Natsuo and Rui start a lengthy relationship until he entered his college year. Rui's one-year trip for her cooking lesson with another man to New York, and Natsuo's experiencing writer's block, put a strong breaking point to their relationship, which ends as soon as she returns, restoring their relationship to brother and sister. Nevertheless, they still care a great deal for one another, Rui especially to Natsuo.
Despite everything when Natsuo finds out about Rui's bad situation, he decides without hesitation to go to New York to support her even telling Hina that, he goes to see Rui without losing time, when he meets her, he gives her a hug being happy to be with her and finally tell her that he still loves her.
Miu is Natsuo's Club President; a fellow aspiring novelist. He briefly thought of her as a potential lover, but they get along as friends.
Momo is Natsuo's fellow club mate although she fell in love towards him since Natsuo was the only man who respected her. However, after finding out that he already set his eyes for Rui, Momo eventually gave up her feelings.
Alexander "Al" Matsukawa Edit
Alexander is a fellow club member of Natsuo and their relationship is mixed with conflicts due to their rivalry for Rui. Still, their friendship was one of a kind since their first encounter is filled with trouble, and Natsuo, in turn, introduced him to the Literature Club in which Rui was also a member.
Reiji is Natsuo's high school teacher and club adviser. To his surprise, Kiriya was his favorite novelist famous by his pen name You Hasukawa. Ever since Natsuo set his eyes to become a novelist, Kiriya guided him by giving reference books rather than an apprentice. He would rather read and evaluate Natsuo's work as an assessment to see if his hard work paid off.
Upon finding out about Kengo stalking Hina, Natsuo develops a deep hatred towards the man. When Natsuo ran in front of Hina to save her from Kengo's attack, he defiantly tells Kengo to stay away from Hina. This incident leads to Kengo's incarceration for his attempted murder of Hina.
After visiting him in prison, Natsuo admits he is unsure of what to feel about Kengo after discovering he was unwilling to admit any wrongdoing.