I love the characters. This could make a good manga. They could just reveal the twist in the first chapter to the readers but drag out the reveal to the occultist, and have a slice of life style comedy where she is trying to hide that she is a demon while keeping her reputation intact, with the jealous girl acting as a sort of antagonist.
They can have the same climax where her true colours are revealed and she crashes out on the occultist. Extra chapters would allow this conflict to develop and showcase each characters' feelings more after the conflict.
Then they could have the same resolution with occultist confronting jealous girl and MC overheating and changing her heart to help her.
Overall though, the oneshot is perfectly fine on it's own with good plot structure, clear conflict and resolution, good pacing, and entertaining characters.