Mitsuki had decided to become a singer two years before, when she was 10 years old. She had made a promise with Eichi Sakurai, then a 16-year-old boy she met at her orphanage, that the next time they met, they would both be closer to their dreams. Eichi wanted to become an astronomer and Mitsuki wanted to become a singer. Shortly after, Eichi was adopted and immigrated to America before Mitsuki could express her feelings for him. Mitsuki hopes that by becoming a famous idol, Eichi would be able to hear her sing and realize her feelings for him.
But Mitsuki’s dreams seem impossible to achieve because she is afflicted with “Sarcoma”, a disease which is curable only through a surgery that could ruin her vocal cords, and destroy her ability to sing. The tumour in her throat already affects her ability to breathe well and sing loudly. And then, one day she is visited by two shinigami, Takuto and Meroko. Because Mitsuki could see them, they inadvertently tip her off that she has only one year left to live.
Mitsuki then realizes she cannot wait any longer to fulfill her dream, so she runs away from home and the shinigami, to try to audition for a singing competition. However, the shinigami stop her before she is able to audition. She moves Takuto, the male shinigami, to agree to a compromise: If Mitsuki promises to go quietly when her year is up, he would help her become a singer, so she could leave the world with no regrets. Takuto gives her the ability to transform into a completely healthy 16-year-old, so that she could meet the age requirements of the audition, and sing without hindrance.
Despite heavy competition, Mitsuki wins over the judges with her excellent voice and her enthusiasm for singing, sealing a contract with Seed Records. To conceal her true identity, she chooses the stage name "Fullmoon" ("Fullmoon," a single word, which is a name, in contrast to "Full Moon" such as in the series title, which refers to the actual moon).
