Chihaya Hiiragi assists her mother at their family's Angel Kiss café. She encounters Natsuki, a magical creature from Evergreen seeking five stolen seeds. These seeds bind to humans, amplify desires, and transform them into monsters. After rescuing Natsuki's injured animal form and nursing him to recovery, Chihaya joins the mission.

Her connection with Natsuki grants Chihaya the ability to transform into Magical Warrior Carmein. This form gives her a taller stature, mature physique, and red hair instead of brown. As Carmein, she wears a distinctive pink qipao-like outfit and wields a wand to purify seed-possessed humans, restoring them without memory. Transformation varies by adaptation: the original visual novel and OVA require a kiss with Natsuki, while the anime uses golden marbles.

Chihaya battles seed monsters while navigating relationships. Complications arise with other warriors like Cerulean Blue (her friend Sayaka Mizushiro) and transfer student Emi Kojima, revealed as Septem from Evergreen. Chihaya evolves from initial reluctance to dedicated acceptance of her role.

Her background reveals her father, believed dead before her birth, is Bergamot—a fugitive from Evergreen who stole the seeds under antagonist Fennel's influence. Bergamot returns to Earth with Septem to protect Chihaya from Fennel. This explains her dreams of Evergreen and her mother's familiarity with the conflict, forcing Chihaya to confront her heritage and her father's motivations.

Chihaya combines kindness with determination. Adaptation differences exist: the anime depicts a cooperative bond with Natsuki, while the OVA shows greater tension. Similarly, her interactions with Cerulean Blue shift from rivalry to teamwork during battles. These dynamics highlight her growth from ordinary student to protector balancing dual lives.