Fū Nonoyama, who transforms into the magical girl idol CompatoLilly, is the younger sister of Rui Nonoyama and the primary protagonist of the series. She is an elementary school student whose birthday falls on June 16, making her a Gemini, and her blood type is O. She has bright and innocent qualities that naturally draw attention from those around her, and she possesses a straightforward personality that leads her to become excited and act immediately whenever something fun or interesting appears. This tendency makes her the center of attention in most situations, whether at school or during her idol activities.

The series begins with Fuu feeling a growing emotional distance from her older sister Rui. The two were once very close, but after Rui stopped singing due to an illness years earlier, a quiet rift developed between them. Fuu misses the sister she used to know and feels lonely about the change in their relationship. This longing becomes the driving force behind her most important wish. While trying to find someone to play with before summer break, Fuu encounters a mysterious spaceship that only she can see. After a dangerous fall, she is brought inside the vessel and meets Meeter, a Felix Star spirit, who grants her magical powers. There are two strict conditions attached to these powers: they will only last for one year, and she must never reveal the existence of magic to anyone, including her family.

Fuu uses a magical item called a Glass Heart to transform into her adult magical girl form, taking on the stage name CompatoLilly. Her magical device is also known as Startto NeoNeon, which she can invoke to help make things happen, though the magic does not always work exactly as intended and can sometimes backfire. Shortly after gaining her powers, she ventures into the city with her magical cat companion Uguisu, where she catches the attention of Toko Kandachi, an aspiring idol talent scout. Toko recruits Fuu into the agency Production Once More, and Fuu makes her debut as CompatoLilly at an event called the Shibu-Yeahhh Teens Meeting, dazzling the audience with her singing enhanced by magical tricks. She quickly gains a large following and becomes a popular idol.

The central irony of Fuu's role in the story is that she and Rui are both magical girls working as idols for the same agency, but neither knows the other's secret identity. Fuu watches Rui's idol debut as MashuLulu on television and even notices that the singing voice sounds familiar, but the magic secrecy rule prevents either sister from discovering the truth. Much of the series revolves around the sisters unknowingly working alongside each other, supporting one another while keeping their magic hidden, and slowly growing closer again through their shared but unrecognized connection. Fuu's childhood wish was for Rui to become her old self again after the illness stole her voice, and becoming an idol is her way of reaching that goal without knowing that her sister is already pursuing the same path.

In terms of relationships, Fuu is closest to her sister Rui, even though they have drifted apart at the start. Uguisu is her constant companion and magical partner. Toko Kandachi serves as her manager and mentor in the idol industry, and she also works alongside fellow idol Sena Aozono. She and Rui eventually form the duo unit LuluttoLilly together under the Production Once More label. Through her experiences as CompatoLilly, Fuu grows from a lonely elementary schooler into a confident performer who gradually begins to bridge the gap between herself and her sister. Her journey involves learning that magic is not a simple solution to everything and that understanding, communication, and courage are just as important as any spell.