A 12-year-old girl who dreams of becoming a pop singer but a tumor in her vocal cords restricts her ability to sing. She is an orphan who grew up in an orphanage, then later went to live with her strict grandmother, Fuzuki, who forbid her from singing. When Mitsuki learned that she would die if her vocal cords were not removed, she refused the surgery, saying that she would rather die than not be able to sing. With Takuto's help, she can transform into Fullmoon (フルムーン, Furumūn?), a 16-year-old pop idol, completely free of cancer and able to sing. As Fullmoon, she is discovered by Seed Records during an audition and releases several singles in both the manga and anime; the anime ending theme songs are all supposedly sung by her. Mitsuki chooses the stage name Fullmoon both because mitsuki means "full moon" in Japanese and because her childhood friend, Eichi Sakurai, wants to become an astronomer and the name reminds her of him.
Mitsuki wants to become a singer because she wants to tell Eichi that she loves him.