Kuze Shuuichi is Asou Renji's neighbor and a good friend to Himura Yuu. He is a womanizer and especially fond of Asou Sumire, despite her being married. Shuuichi is a famous violinist of whom Hirono Hiro is a fan. In the past, he went to the same school as Himura Yuu, Hirono Nagi, and Amamiya Yuuko; all of whom he considered to be his friends, though Kuze was planning on moving to another country by the time he met the last one.

Being a ladies man, Himura asked Shuuichi to investigate his kouhai (underclassman) Yuuko, which turned out with results, as she was the one scaring her classmates and the one who started a fight (to keep what is in her bag hidden). Before leaving, Kuze Shuuichi had a farewell party, but only inviting his three close friends at the school. However, due to their problems, none of them stayed through his entire piece (Amamiya felt his playing was too wonderful, which made her leave in tears, with Himura following her at Shuuichi's insistance), and Nagi did not even come. Later, the violinist came back to for a short time to visit an acquaintance. During this time, he sent a letter to Yuu and Yuuko, who were living together, telling them his news. While he was playing in the street, a little girl, named Hayama Mizuki, also known as Miki, was drawn to the melody from afar. When she reached Kuze, he called her a "cute little lady (kawaii ujouchan)" and tied his own hair ribbon on her head, which made Miki happy.

Six months before Mizuki went to Otowa, Australia for vacation prior to entering university, Kuze was diagnosed with a fatal disease. During this period, he decides to end all of his relationships, including the one with his fiancee, as he does not want anyone to miss him. He also wants his neighbors, Renji and his mother, Sumire, to treat him normally. While burning the letters he received, Kuze finds a rare one from Hirono, but decides to burn it as well without reading the content. As he is emptying his place of furniture, he meets Hayama, who wants someone to play with, but her cousin, Asou, is busy studying for examinations. Oblivious to Shuuichi's condition, Mizuki asks him to play for her live (he has albums), but he lies, saying that the violin is broken, where in fact he has no desire to play. After she finds out about his illness, Kuze leaves Hayama, like he did to the other girls he dated. Depression and frustration take over Shuuichi, leading him to fight with Yuu and Nagi. Moreover, the latter lies to Miki, telling her that she is Shuuichi's fiancee, and that Hayama should leave him alone. In spite of that, thanks to Himura's advice and encouragement from her friends, including receiving two school rooftop keys, Mizuki picks herself together and decides to fight (literally) for Kuze. Just as she was maing up with him, Shuuichi collapses and is hospitalized. Though there is a high risk with low gains, he decides to finally do something in which Kuze might lose at (which made him abstain from confessing to Hirono, whom he liked, in the past) and have the operation. It ends in success, and Shuuichi lives his remaining days with his lover Mizuki (who comes to call him by his first name), appreciating every second he has with her, and hoping to see her grow as much as possible.