Momoka Kawakabe arrives in Kamitsumihara devoid of any memory of her past or identity, an enigmatic figure appearing suddenly within the story's setting. She finds a place within the Kamiazuma clan household, where her most immediately striking characteristic is an apparently boundless appetite; she is constantly eating, preparing, or dreaming about food, demonstrating remarkable puppy-like accuracy in catching morsels with her mouth.

Her personality radiates bright energy and unfiltered emotional expressiveness, forming a stark contrast to Tōka Kamiazuma's gloomy withdrawal, the clan member she develops a deep infatuation for. Despite Tōka's consistent harsh rejections, she displays unwavering resilience, persistently seeking his company and deriving pure joy from simply being near him. This resilience is underscored by her ability to instantly shed sadness, often just by leaning on Tōka once. However, her simple-minded nature and impulsive actions frequently lead to mishaps that provoke her classmates' anger.

Unbeknownst to her or others initially, Momoka is the human incarnation of the goddess Sei, awakened from a long slumber. Her awakening and arrival at the Kamiazuma household are intrinsically tied to an ancient legend and a cycle of tragedy involving the soul-consuming sacred blade known as the Stone Sword. Her core purpose as Sei is to save the soul of her brother, the Stone Sword's former wielder, Isamihiko. Hints surface suggesting that she, as Sei, was responsible for killing Isamihiko in the distant past.

Her existence is ultimately revealed to be a creation within the writings of Yumiko Kamiazuma, confining her true reality as Momoka solely to Kamitsumihara. As the goddess Sei, she holds the crucial role in the ritual necessary to break the ancient curse plaguing the land. Performing this ritual demands her sacrifice and would result in her disappearance alongside her sister goddesses. Despite this consequence, driven by profound love for Tōka and determination to end the curse's suffering, she chooses to fulfill her role. She makes a final vow to Tōka: if they meet again in any future existence, she will not hesitate to ensure their love is realized.