In the manga, a demon called "shinki" lives inside each person. Kei, a high school student, hates his shinki, and goes about life trying to suppress his inner demon. But a certain event brings out the shinki inside him.

(Source: ANN)

"...a lonely woman hires her favorite novelist - who has writer’s block - to write her a love and a family… and, in the process, they become both to each other." (Okazu)