Koi wa Hito no Hoka恋は人の外 (Japanese); 恋人是人外 (Chinese); Love is for Other People; Love is for Others (English)

Tsukumo Sayo made a promise with a child demon when she was 5 years old. Now, 10 years later, that demon is tired of waiting her and suddenly he appears in front of Sayo to fullfil that promise? "I'll definitely make you fall in love with me". Well now, which side will fall first?